THE MAMMALIAN EGG 



GENERAL BIOLOGY OF EGGS 



Discovery 



'Omne vivum ex ow'— 'All living things come from eggs' — was a 

 conclusion reached several centuries ago by the anatomist William 

 Harvey (165 1), better known for his discovery of the circulation of 

 the blood. As a generalization, it has proved remarkably true, for 

 there are few forms of life that arise exclusively by other means 

 and these are to be found chiefly among the single-celled organisms. 

 The generalization is remarkable also because it was made when the 

 nature of eggs of any sort was most imperfectly known and before 

 those of mammals had even been properly identified. At that time, 

 what were termed mammalian 'eggs' took most diverse forms: 

 spherical or ovoid objects, filamentous or membranous structures, 

 and coagulated masses. These 'eggs' were considered to have been 

 developed within the uterus from the mingled male and female 

 'semen'. Galen (a.d. 130-200) had introduced the idea of female 

 'semen' as a substance separated from the blood stream by the 

 ovaries and passed into the uterus through the Fallopian tubes. 

 Later, de Graaf (1672) homologized the mammalian ovary with that 

 of the bird, maintaining that the eggs originated here and then 

 passed into the uterus ; he believed that the ovarian follicles, which 

 now bear his name, were either the eggs themselves or else contained 

 something analogous to eggs. The former possibility appeared to 

 be supported by the similarity in general form between the follicle 

 and the uterine 'egg' — de Graaf worked with rabbits, in which the 

 blastocyst is a spherical body of about the same size as the pre- 

 ovulatory follicle. He had also observed how, in the rabbit, the 

 follicle becomes radically altered after coitus and, a few days later, 

 blastocysts can be found in the uterus. The Fallopian tubes, how- 

 ever, were manifestly too narrow to permit the passage of objects 

 of this size and so de Graaf seems to have preferred the view that the 



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