10 THE DEVELOPMENT OF THE CHICK 
nized in the tail, viz., the connecting piece (pars conjunctionis) 
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 
f 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- 
Ku brane or zona radiata without reference to its 
Fic. 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 
in most 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 
