Evaluación de los resultados de la Planificación

2.1 Earliest theories

Through the seventeenth century, observations into embryology were made by anatomists Hieronymus Fabricius ab Aquapendente (1537-1619) (De formatione foetus, 1604), William Harvey (1578-1657) (Exercitationes de generatione animalium, 1651), and physician Marcello Malpighi (1628-1694) (De formatione pulli in ovo, 1673) for example.

Italian physician Marcello Malpighi demonstrated that the microscope was useful for embryology, anatomy, and physiological studies, greatly extending knowledge regarding structure and function of the human body. Malpighi’s work was known not only in Italy, but was popular with the Royal Society in England⁵⁸. Another member of the Royal Society, Robert Hooke (1635-1703), developed his microscope in the 1660s, writing a book, Micrographia (1665), based on his observations. Amongst items described in Micrographia is cork, which Hooke described as comprising “cells” or “little Boxes”⁵⁹, comparing them to the ‘cells’ of monasteries, where monks would sleep.

A significant discussion of the seventeenth century was between the ovists and spermists. In 1678, Dutch textile merchant-turned microscopist, Anthoni van Leeuwenhoek (1632-1723) identified spermatazoa. Leeuwenhoek, and those who followed his work, believed that the sperm was the germ, considering the egg a nest required for ‘hatching’. The opposing belief was that the egg was the true germ. The ovists (or ovulists) believed that the sperm were relatively insignificant⁶⁰.

58 Lancaster, 2014 p 29-30

59 Hooke, 1665.

60 Harris, 2000.

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A second discussion point of the period concerned the epigenesists and the preformationists. The epigenesist argument was that the development of an egg or embryo was an entirely new construction or creation. The preformationists argued however that embryonic development was akin to a flower bud unfurling: the embryo was preformed. As Harvey remarked in Exercitationes de generatione animalium,

“the vegetal primordium whence the fœtus is produced...pre-exists”⁶¹. This latter approach was favoured by noted microscopists Malpighi and Jan Swammerdam (1637-1680), as well as Albrecht von Haller (1708-1777) and Charles Minot (1852-1914)⁶².

2.2 Experiments in development (to 1800)

Neither the epigenesis/preformation nor ovist/spermist debates were resolved in the seventeenth century; little was achieved in the eighteenth either. It is likely that this is due to the lack of experimental progress that was made in embryology, which in turn is a demonstration of slow technological progress during this period. This said, some researchers were still concerned with developmental biology, however no works really surpassed the excellent studies carried out by Malpighi a century prior.

One individual who did concern himself with embryology was Haller, who in the mid-1700s, carried out dissections on animals post-mating. Focusing on larger mammals, such as sheep, Haller would claim that he could see nothing for the first fortnight post-mating, and that only a fluid substance could be observed in the uterus.

This fluid, Haller claimed, ‘curdled’ (gerinnen)⁶³, forming the embryo. Haller’s conclusions were considered to be plausible and the most reliable explanation, leading his theory to be taught in universities across Europe through the late eighteenth century⁶⁴. There was an opposing theory available however. In 1797, Scottish anatomist William Cumberland Cruikshank (1745-1800) published observations he had been making since the 1770s. Cruikshank dissected rabbits after mating. He reported seeing ova three days post-mating in the oviducts, and four days post-mating in the uterus. Cruikshank made significant numbers of observations which were eventually published in the Philosophical Transactions of the Royal Society; Sarton

61 Harvey, 1847 p 465.

62 Harris, 2000.

63 Translation by Sarton, 1931.

64 Harris, 2000.

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suggested however that there was such faith in Haller’s theory that Cruikshank’s work was given little consideration⁶⁵.

2.3 Fertilisation to embryo (1800-1840)

Formation of the chick embryo had been studied for centuries; one of the earliest works describing such observations is De generatione animalium by Aristotle.

Despite this, no connection had been made between early embryonic development and (what we now understand to be) cells. So far, there was also no evidence to show that there was any correlation between the embryonic development of mammals and the development of birds, for example.

Jean Louis Prévost (1790-1850) and Jean Baptiste Dumas (1800-1884) described the furrowing of frog eggs after treating them with fluid expressed from frog testicles⁶⁶. For them, the segmentation occurred only after fertilisation.

Although initially they only appear to describe the changes at the surface, their comparison of the dividing egg to a raspberry suggests that Prévost and Dumas understood that this was not only a surface phenomenon.

Mauro Rusconi (1776-1849), whilst at the University of Pavia, built on the work of Prévost and Dumas, describing in detail the ‘segmentation’ of the egg post-fertilisation. Rusconi carefully described ‘furrows’ on the surface, which he observed to eventually result in division and subdivision, creating ever-smaller units.

Rusconi’s comprehensive descriptions (and significant experimental detail) indicate that he clearly understood the process that was occurring⁶⁷. For instance, after these first subdivisions, Rusconi described the development of une masse granuleuse.

Rusconi published his observations in 1834, although he had started his studies in 1826. Like Prévost and Dumas, Rusconi also made it clear that he believed segmentation occurred post-fertilisation; in Karl Ernst von Baer’s (1792-1876) work (which mostly ignored Rusconi’s findings), he theorised that the segmentation previously described by Prévost and Dumas occurred before fertilisation. von Baer believed this process occurred in order to give all parts of the egg access to the sperm.

65 Sarton, 1931.

66 Prévost and Dumas, 1824.

67 Harris, 2000.

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2.3.1 Identification of ‘embryonic cells’

English physician Martin Barry (1802-1855) was the first to identify the individual cells of the early embryo, comparing them to cells of the adult. In three papers titled Researches in Embryology published in 1838, 1839, and 1840 in the Royal Society’s Philosophical Transactions, Barry described the mature egg, fertilisation, and the early development of the mammalian embryo in a fashion that makes it clear he believed that the subdivision of the fertilised egg was equivalent to cells observed and described in adults.

In Researches in Embryology: First Series (1838), Barry began with a brief history, considering Regnier de Graaf’s (1641-1673) theory that the ovum existed pre-formed in the ovary, and Haller’s opposition⁶⁸. Later, Cruikshank would support de Graaf’s theory, but lacked the evidence to be taken particularly seriously. Evidence was collected by Prévost and Dumas, and later von Baer (as noted above). Barry himself went to Germany to work with Johannes Müller (1801-1858) and his students to learn about animal development and microscopy (see section 3). The skills Barry learned enabled him to dissect and section mammalian ovaries; the First Series described his observations regarding ova development, maturation, structure and size.

Barry believed that the germinal vesicle was formed first.

The Second Series (1839) focused on development of the ovum, tracing the early stages of development. Barry noted that there was still a “dark period” (between mating and appearance of vertebrae) in mammalian development - little was understood regarding this time, and Barry aimed to shed some light⁶⁹. To help him, Barry used rabbits (although one of the carefully drawn figures also includes the ovum of a tiger!). Barry claimed to have examined hundreds of ova, both through dissections and preserved samples, carefully measuring and drawing what he saw.

The figures in Barry’s paper clearly reflect this attention to detail.

68 Here, it is likely that Barry is referring to Haller’s earlier views. As a student, Haller followed the teachings of Herman Boerhaave (1668-1738), whose lectures he attended in Leiden (1725-1727); Haller therefore supported preformationism, with a bias towards the male. In the 1740s, Haller switched his allegiance, believing that the epigenesis theory was the more likely explanation. Haller changed his mind for a third time however, having carried out work on chicken eggs. For the latter years of his life, Haller became an ovist preformationist. This suggests that Haller would have supported de Graaf’s theory that the ovum exists pre-formed in the ovary. (For a more detailed discussion of Haller’s thoughts on embryology, see Roe, 1975.)

69 Barry, 1839 p 307.

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Barry described his stages of development in intricate detail alongside his figures. For example, in the sixth stage of development, Barry described the second set of cell divisions:

“The centre of this fluid was occupied by four large vesicles. These vesicles were spherical, but somewhat flattened…Some of these vesicles presented in their interior a minute pellucid space, which may possibly have been a nucleus”⁷⁰.

A note with the figure suggested that Barry observed a nucleus in all similar vesicles he observed. The seventh and eighth stage resulted in more vesicles, although smaller. By the ninth stage, Barry stated that the small vesicles hung together like a

“mulberry”⁷¹. At the tenth stage, there were even more vesicles, and within each was

“an object resembling the ‘germinal vesicle-like nucleus’ observed by Valentin in

‘globules’ from various parts of the nervous system”⁷². Here then, Barry is clearly indicating that what he is seeing in the developing embryo can be directly compared with the adult cells observed⁷³. By doing this, Barry is establishing that what he sees at the embryonic level is analogous to the ‘subunits’ of adult animals⁷⁴. Although Barry refered to ‘vesicles’ as opposed to cells, I do not think this is particularly significant; the term Zellen only came into common use following publication of Theodor Schwann’s (1810-1882) 1839 monograph popularising Cell Theory (see below). Prior to this, ‘vesicle’ was a term used to describe what we would now consider to be ‘cells’ (in both plants and animals).

Carl Bergmann (1814-1865), whilst working as Rudolf Wagner’s Assistent in Göttingen, would also describe furrows in a similar fashion to Prévost and Dumas.

Bergmann’s work studying the eggs and embryos of newts in the late 1830s and early

70 ibid p 323.

71 ibid p 324.

72 ibid p 324-5.

73 Barry is referring to Gabriel Gustav Valentin (1810-1883), a German physiologist, known particularly for his descriptions of cells in nervous tissue.

74 A further indication that Barry understood the ‘vesicles’ of the early embryo as those which would become the ‘vesicles’ of adulthood is in the discussion of methods. Barry utilised the most modern techniques available for his observations, which were primarily histological methods. For example, in the Second Series paper, Barry described using ‘kreosote water’ for preserving ova. This is a solution Müller had shown to Barry, which he used to preserve tissues of the nervous system. Barry must have believed that the ‘substance’ of the ovum must be similar to the ‘substance’ of nervous tissue to believe that Müller’s kreosote water would be as useful for preserving ova as it was nervous tissue.

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1840s made him conclude that the ‘furrows’ observed were cell divisions, and that these cell divisions gave rise to the cells of the embryo.

Shortly after Bergmann’s observations were published, H. Bagge published his dissertation⁷⁵. In his work, Bagge claimed that the successive segmentation of the egg eventually gave rise to the cells of the embryo in nematode worms. Furthermore, Bagge described the division of the nucleus prior to division of the cell in Ascaris.

Professor of zoology and anatomy at the University of Königsberg, Heinrich Rathke (1793-1860), came to similar conclusions as Bagge, although Rathke studied other invertebrates. Rathke’s work was published just a few months later in 1842. As opposed to ‘furrows’, Rathke preferred the term Durchfurchung (‘cleavage’), when describing the segregation of the egg. Such a term suggests that Rathke wanted to make clear that this was not only a surface phenomenon. Likewise, German embryologist Adolf Grube (1812-1886) also used the tern Durchfurchung in his description of division of the fertilised leech egg⁷⁶; this has been translated to

“fissures”, leading to clear division of the fertilised egg into four blastomeres⁷⁷.