224 



THE HUMAN SPECIES 



ently still retain traces of our primordial birthplace, a shore 

 washed by the tides." l 



The human ovum was discovered in 1827 by K. E. v. 

 Baer. It is essentially identical with that of any other verte- 

 brate or invertebrate, being composed of a globule of pro- 

 toplasm with nucleus, nuclear framework and nucleolus. 



In microscopic details characteristic differences can be 



FIG. 112. A ripe human Graa- 

 fian follicle. (Haeckel.) a, 

 the mature ovum ; b, the 

 surrounding follicle - cells 

 ("discus proligerus ") ; c, 

 epithelial-cells of the fol- 

 licle (" membrana granu- 

 losa ") ; d, the fibrous 

 membrane of the follicle 

 (" tunica fibrosa ") ; e, ex- 

 ternal surface. 



FIG. 113. The human ovum after 

 issuing from the Graafian 

 follicle, surrounded by the 

 cells of the discus proligerus 

 (in two radiating crowns). 

 (Nagel.) z, zona pellucida 

 (" ovolemma ") ; /, proto- 

 plasm of the cell - body 

 (" cytosoma ") ; k, nucleus 

 of the ovum (" embryonic 

 vesicle "). 



recognised as regards the size and structure of the protoplasm, 

 the breadth of the zona pellucida and the size of the nucleus. 

 The diameter of the complete ovum is O'i8-o'22 mm., the 

 nucleus (embryonic vesicle), o'O4-o'O5 mm., the nucleolus 

 (germinal spot), O'oc^-O'oo/ mm. 



1 Darwin, Descent of Man (Murray, and edition, 

 p. 248. 



), vol. i., p. 10 and 



