10 



THE DEVELOPMENT OF THE CHICK 



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

 to the neck, frequently called the middle piece, the main piece (pars 

 principalis) and the end-piece or terminal filament (pars terminahs). 

 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 different 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 

 theoretical mode of origin. 



The ovum escapes from the ovary (ovula- 

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

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



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 



Fig. 1. — Sperma 

 tozoon of the pig 



