GERM CELLS AND THEIR FORMATION 



109 



the Holocephali (Dean) (Fig. 55), which often remain intact and func- 

 tional, in their passive way, for more than a year. In some cases the 

 egg membranes may have a nutritive value and may augment or re- 

 place the food supply in the form of yolk ; the eggs 

 of birds and snails are good illustrations of this 

 relation. 



We must now trace briefly the steps leading 

 to the formation of the ova and spermatozoa 

 as the highly specialized cells we have de- 

 scribed. In the lowest Metazoa, Sponges and 

 some Hydroids, the germ cells are scattered 

 through the tissues of the organism as sepa- 

 rate, free cells, which may migrate from place 

 to place, feeding and growing, often at the 

 expense of the other cells (Fig. 56). But in 

 other Hydroids, and in all forms above these, 

 the germ cells are localized in a definite re- 

 productive tissue and organ, or series of 

 organs, the gonads ovaries and testes. The 

 simplest gonads are merely masses of rapidly 

 proliferating cells (Fig. 57), usually bordering 

 a cavity which is the coelom, and which is 

 supposed to be primarily this reproductive 

 cavity simply. In most of the higher forms 

 the coelom comes to have many secondary re- 

 lations, and forms in addition to the repro- 

 ductive and other smaller cavities, the very 

 extensive body cavity. In the embryos of 

 the Craniates there is a pair of longitudinal 

 ridges, either side of the attachment of the Fl ?- f?;""^ 



capsule of the Holo 



dorsal mesentery, through a considerable ex- cephaian, Chimera 



tent of the body cavity; these are the genital 



ridges, and the peritoneum covering these be- natural size. After 



comes thickened by the enlargement and pro- 



liferation of the cells (Fig. 58). These are the rudiments of 



the gonads. The cells composing these rudiments are often of 



two kinds. Some of them, indifferent cells composing in gen- 



