GENEEATIVE OEGANS OF VEETEBEATA. 609 



cell becomes an egg, wMle the rest form a cellular layer around it — 

 tlie follicular epithelium, which unites with the surrounding con- 

 nective tissue to form the ovarian follicle. Each invagination of the 

 germinal epithelium either forms a single follicle, as in the Anamnia 

 (Selachii), or these groups of cells grow out and form the rudiments 

 of a number of follicles, as in the Craniota. 



The cells of the ovarian follicle that are set around the egg 

 generally remain indifferent, and aid in the nutrition of the egg as 

 well as in the formation of the yolk-sac, which surrounds it. The 

 egg itself, and the cells of the follicle which surround it, undergo 

 more or less considerable modifications. When the egg and the 

 follicle increase equally in size, the follicle-cells form a simple epi- 

 thelial layer, as in Fishes, Amphibia, Kep tiles, and Birds. But in 

 the Mammalia they multiply while the egg-cell remains relatively 

 small, and for a long time they fill up by far the greater part of the 

 follicle. As this follicle grows a cavity is gradually formed in its 

 interior which is filled with fluid ; this causes the cellular layer of 

 the follicle to be extended around its wall (membrana granulosa), 

 while at one point, which is somewhat thickened, it encloses the egg. 



The changes which obtain in the egg-cell relate to the yolk, 

 and they are accompanied by an increase in the size of the egg. 

 This may be seen in the Teleostei, where the granules of the yolk 

 often undergo great metamorphoses. The same happens to the eggs 

 of the Amphibia. In the Selachii, Eeptilia, and Aves the yolk- 

 granules are greatly increased in number, and are specially differ- 

 entiated. Owing to the number present the ripe egg is of a 

 considerable size. 



The region invested by the germinal epithelium is the point 

 at which the male germ -glands are also developed, but it 

 seems that this epithelium does not take any direct share in the 

 formation of the testes. The earliest differentiation of the glandular 

 tubes (seminal canals), which make up the testes, has not yet been 

 observed ; the view that they are formed from a portion of the 

 primitive kidneys is beset by the difficulty of their having no 

 relationship of any kind with these organs. 



The form-elements of the sperm are developed by the dif- 

 ferentiation of the epithelium of the seminal canals. In all Verte- 

 brata these are movable filaments which are given off from a thicker 

 portion of varying form — the so-called head. This head is dis- 

 coidal or elliptical, as in many Mammals and Fishes, or elongated, 

 as in the Selachii, Amphibia, and Aves. In the latter it is fre- 

 quently coiled in a corkscrew fashion. The seminal filament of 

 some Amphibia (S alamandrina and Toads) is distinguished by an 

 undulating membrane. 



§ 451. 



The germ-glands are developed from the structures known 

 as genital ridges. Sometimes more and sometimes less of this ridge 

 is converted into the ovary or testis. The simplest condition is seen 



2 R 



