10 THE DEVELOPMENT OF THE CHICK 



nized in the tail, viz., the connecting piece (pars conjiinctionis) 

 next to the neck, the main piece (pars principalis) and the end- 

 piece or terminal filament (pars terminalis). The entire tail is 

 traversed by an axial filament; in the region of the connecting and 



main pieces the axial filament is surrounded by 

 a protoplasmic sheath (involucrum) which may 

 be variously modified in different animals. The 

 end-piece is made up of the axial filament 

 alone. 



The Ovum. The ova of difTerent phyla and 

 classes of animals vary greatly in size, in or- 

 ganization, and in the nature of their enve- 

 lopes. In considering these variations we shall 

 limit ourselves to the vertebrates. Within the 

 ovary the ovum receives two envelopes, viz., a 

 primary envelope, the so-called vitelline mem- 

 brane, which is supposed to be secreted by the 

 ovum itself, and a secondary or follicular mem- 

 brane, which is secreted by the follicular cells. 

 (See Chap. I). Theoretically the distinction be- 

 tween vitelline membrane and follicular mem- 

 brane (primary and secondary egg-membranes) 

 is perfectly clear; but practically it is impossi- 

 ble in most cases to make such a distinction. 

 Therefore the membrane that surrounds the 

 ovarian ovum will be termed the vitelline mem- 

 brane or zona radiata without reference to its 



YiQ_ 1. Sperma- theoretical mode of origin. 



tozoon of the pig- The ovum escapes from the ovary (ovula- 

 eon from the vas tion) by rupture of the wall of the follicle, and, 

 deferens. (After -^^ ^^g^ vertebrates, is taken up by the oviduct 

 Ballowitz.) through which it passes on its way to the ex- 



terior. Within the oviduct it may become surrounded by tertiary 

 membranes secreted by the wall of the oviduct itself. Tertiary 

 membranes are lacking in some vertebrates, in others they are 

 of great importance. Thus in birds the albumen, the shell- 

 membrane and the shell itself are tertiary membranes. 



The principal differences to be emphasized in the ova of ver- 

 tebrates are, however, in the amount and arrangement of the 

 yolk contained within the ovum proper. All ova contain more 



