DEVELOPMENT OF THE GENERATIVE ORGANS. 707 
Nassonow [848], 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 6m to Iom 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. 
