DEVELOPMENT OF THE GENERATIVE ORGANS. 707 



Nassonow [348], from an examination of the rudimentary 

 testes in Lepisma saccharina, concluded that the ducts closely 

 resemble segmental tubes. The great similarity of the rudi- 

 ments of the sexual glands of insects with the rudimentary 

 nephridia of Vertebrates, the manner in which the tissue forming 

 the neck of the ovary becomes converted into irregular tubules 

 and the origin of the egg-stocks, lend, I think, additional prob- 

 ability to this view. 



g. On the Nature of Ova and the Relations of the Germ to 



the Vitellus. 



General Observations on the Nature of the Ovum. The ovum of 

 almost every animal, from the highest to the lowest, is known 

 to consist, in its earliest stage of development, of a single cell; 

 this cell, the primordial ovum, exhibits a distinct nucleus, which 

 subsequently becomes vesicular. 



The large vesicular nucleus of the ovum was termed by its 

 discoverer, Purkinje, the germinal vesicle. In young ova it is 

 always present, and has a very characteristic appearance. It 

 consists of a transparent nucleus, usually from 6/u- to 10 JJL in 

 diameter, and contains a single highly refractive nucleolus, the 

 germinal spot (macula germinativa, Wagner). In older ova it 

 enlarges rapidly, and is seen to consist of a nuclear membrane 

 enclosing a clear material or matrix, embedded within which 

 are strands of nucleoplasm, enclosing one or more well-marked 

 nucleoli. Frequently there is but one nucleolus, the germinal 

 spot. 



In its simplest form the ovum retains its original condition, 

 and consists of a cell, with or without a special envelope. The 

 cell substance is a slightly granular protoplasm, enclosing the 

 germinal vesicle, and is termed the germ yelk; after the im- 

 pregnation of the ovum it undergoes cleavage throughout, 

 the nucleus dividing previously to the cell, so that the entire 

 ovum divides successively into 2, 4, 8, 16, 32, and 64, or more 

 cells, from which the embryo is developed. Such ova are 

 termed holoblastic. 



