6. The Discovery of the Egg in Higher Eukaryotes

From: The 7 Sexes

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6 The Discovery of the Egg in Higher Eukaryotes While it was known to almost all of humanity since antiquity that birds layeggs,itwasnotknownthattheeggisacelluntilcelltheorywasdeveloped in the 1830s. In 1652, William Harvey (1578–1657) found two things of interest when studying reproduction in the fallow deer.1 One was that after copulation and conception in the fall, the gestation process of the fallow deer goes into an arrested state for several months. The process is called diapause and it allows seasonal regulation of when birth takes place. It occurs during the blastocyst stage and was observable to Harvey as a small spot that neither went away nor enlarged until the winter. Harvey’s other, more significant, observation was that after copulation the uterus of the deer was empty. There was no coagulum to be seen or evidence that anything had changed in the uterus. What Harvey did not knowwashecouldnotseetheblastocystthatimplantsitselfintheuterus of the fallow deer because it is transparent and microscopic. The mammalian blastocyst at implantation at best would be the size of a period in a sentence. It does not enlarge before implantation in the uterus. It receives no external nutrient to grow until it implants. In 1843, Theodor L. W. Bischoff (1807–1882) was able to demonstrate the presence of that blastocyst. Bischoff studied the egg in rabbits, dogs, guinea pigs, and the roe deer, following it through its cleavages, blastocyst formation, and early embryonic development.2 Earlyscientistshaddifficultydescribingthemammalianegg.When ReinierdeGraaf(1641–1673)describedwhatistodaycalled,inhishonor, the Graafian follicle, he thought he was describing the egg,3 but the actual egg is within the follicle. De Graaf’s confusion was caused by his choice of organism: he was studying rabbits. A rabbit’s Graafian follicle The Discovery of the Egg in Higher Eukaryotes 35 is about the same size as its blastocyst, and de Graaf was working in the era before there were good microscopes. Because the follicle seemed too big to pass through the oviduct, de Graaf assumed that it was pulverized during its passage and reconstituted when it emerged and rested in the uterus. The oldest printed illustration of female internal genitalia is in Fasciculo de Medicina (1491) by Johannes de Ketham, a Viennese physician. Little is known about the life of de Ketham, who was known as Johannes von Kirchheim before he moved to Italy, other than his having been active in the 1460s. De Ketham’s view is a fusion of Aristotle’s and Aquinas ’s view of reproduction: “Concerning conception of the fetus: in the first month coagulation . . . of the blood takes place; in the second, the body forms; in the third, there is the binding of the soul with the body.”4 In 1561, Gabriele Fallopio (Fallopius) (1523–1562), who was a student of Andreas Vesalius, first described the oviduct, which is also called the Fallopian tube. In turn, his student, Hieronimus Fabricius (1537–1619) believed that the oviduct was the source of the secretions that produced the yolk and albumin found in chicken eggs. It was Fabricius who got his student William Harvey interested in studying the egg. In 1688, de Graaf also clarified the status of the ovaries: “The general function of the female testicles is to generate the ova and to bring them to maturity, so that they serve the same purpose in women as the ovaries in birds. Hencetheyshouldratherbecalledovariesthantestesbecausetheyshow no similarity, either in form or contents, with the male testes properly so called.”5 William Cruikshank (1745–1800) corrected de Graaf’s theory of egg passage in 1797. By tracing an actual rabbit egg down its oviduct, he discovered that the follicle itself never entered the oviduct.6 This was confirmed by Ernst von Baer in 1827 when he punctured a follicle with a knife point, placed its contents on a glass slide, and found the egg to be exactly the same size, color, and shape as what he (and Cruikshank earlier) had seen in the oviduct. This suggested to von Baer that what de Graaf had observed in the uterus was not the follicle, but the blastocyst of the rabbit embryo.7 De Graaf also observed the conversion of the follicle into a yellowish material called the corpus luteum. There was some dispute among nineteenth-century biologists and physicians about what the corpus 36 luteum was: some believed the material was identical to egg yolk, and others believed it was not. The role of the corpus luteum in maintaining pregnancy and preparing the uterus for implantation of a blastocyst was not worked...