74 



COMPARATIVE PHYSIOLOGY. 



hypoblast, and intermediate less closely arranged cells, termed, 

 from their position, mesoblast. 



It wiU 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. 



THE 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 



i--sp 



V.l 



vt 



tm-^ 



Fig. 73. 



Figs. 73-75.— 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, show- 

 ing the stages of development of the amnion and of the yelk-sac. I, II, in, IV 

 are transverse sections at about the same stages of development, i, ii, ili, pos- 

 terior part of longitijdinal section, to illustrate three stages in formation of the 

 allantois. «, embryo: y, yelk; jrp, plenroperitoneal cavity; vt, vitelline mem- 

 brane of amniotic fold; il, 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 



