46 STRUCTURE OF THE EGG. 



kinds of cells, and not in homogeneous cells by any 

 alleged process of differentiation or transmutation. 



In man and the mammifera, the ovum as it exists 

 in the ovary or egg-bed, though so very minute in 

 size as to be no more than visible as a speck 

 to the naked eye, comprises all the essential parts of 

 an egg, viz. the germinal vesicle, yolk, and yolk 

 membrane. Professor Ernst von Baer, who first 

 discovered the ovarian ovum of man and the Mam- 

 malia, mistook it for the homologue of the germinal 

 vesicle of the bird's egg — which had been discovered 

 a short time before by Professor Purkinje ; but the 

 discovery of a real germinal vesicle in the human 

 and mammalian ovum by Valentin in Germany, 

 Coste in France, and myself in this country, demon- 

 strated the true nature and significance of that body. 

 In the bird the ovum leaves the egg-bed as the 

 full-sized yolk ; whilst by the superaddition of the 

 white and shell around the yolk, which takes place 

 in the oviduct, the whole egg of the bird when 

 laid, is, as is well known, larger still. 



The first observable changes which take place in 

 an ovum as the immediate consequence of fecundation 

 •are : the disappearance of the germinal vesicle, 

 and the cleavage or division and subdivision of 

 the yolk, resulting in the resolution of its substance 

 into the • different kinds of cells composing the 

 blastoderma. 



In the ovum after fecundation, when the germinal 

 vesicle has disappeared, Haeckel thinks is to be 



