19 ANIMAL PHYSIOLOGY. 
termediate less closely arranged cells, termed, from their posi- 
tion, mesoblast. 
It will be noticed that all future growth of the embryo be- 
gins axially, at least in the early stages of its development. 
As the subsequent growth and advance of the embryo de- 
pend on an abundant and suitable nutritive supply, we must 
now turn to those arrangements which are temporary and of 
subordinate importance, but still for the time essential to devel- 
opment. 
_EMBRYONIC MEMBRANES OF BIRDS. 
It will be borne in mind throughout that the chief food-sup- 
ply for the embryo bird is derived from the yelk ; and, as would 
> 
Fie, 77. 
Fics. 77-79.—A series of diagrams intended to facilitate the comprehension of the relations of 
the membranes to other parts (after Foster and Balfour). A, B,C, D, E, F are vertical 
sections in the long axis of the embryo at different periods, showing the stages of develop- 
ment of the amnion and of the yelk-sac. I, II, II, IV are transverse sections at about the 
same stages of development. 1, ii, iii, posterior part of longitudinal section, to illustrate 
three stages in formation of the allantois. e, embryo; y, yelk ; pp, pleuroperitoneal cav- 
ity 5 be vitelline membrane of amniotic fold; al, allantois; a, amnion; a’, alimentary 
canal. 
be expected, the older the embryo the smaller the yelk, or, as it 
is now called when limited by the embryonic membranes, the 
yelk-sac (umbilical vesicle of the mammalian. embryo). The 
manner in which this takes place will appear upon an inspec- 
tion of the accompanying figures. 
Very early in the history of the embryo two eminences, the 
head and the tail folds, arise, and, curving over toward each 
