COMPARISON OF THE GERMINAL LAYERS. 



241 



The food-yolk must be supposed to have 



like that of the Sauropsida, 

 ceased to be de- 

 veloped on the es- 

 tablishment of a 

 maternal nutrition 

 through the uterus. 



On this hypo- 

 thesis all the de- 

 velopmental phe- 

 nomena subse- 

 quently to the for- 

 mation of the blas- 

 todermic vesicle re- 

 ceive a satisfactory 

 explanation. 



The whole of 

 the blastodermic 

 vesicle, except the 

 embryonic area, re- 

 presents the yolk- 

 sack, and the 

 growth of the hy- 

 poblast and then of the mesoblast round its inner wall represents the 

 corresponding growths in the Sauropsida. As in the Sauropsida it 

 becomes constricted off from the en)bryo, and tbe splanchnopleuric 

 stalk of the sack opens into the ileum in the usual way. 



In the formation of the embryo out of the embryonic area the 

 phenomena which distinguish the Sauropsida from the Ichthyop- 

 sida are repeated. The embryo lies in the centre of the area ; and 

 before it is formed there appears a primitive streak, from which there 



Fig. 177. Diagrammatic longitudinal section through 

 the posterior end of an embryo bird at the time of 



THE FORMATION OF THE AlLANTOIS, 



ep. epiblast; Sp.c. spinal canal; ch. notochord; n.e. 

 neurenteric canal; hy. hypoblast; p.a.g. postanal gut; pr. 

 remains of primitive streak folded in on the ventral side; 

 al. allantois; me. mesoblast; an. point where anus will be 

 formed; p.c. perivisceral cavity; am. amnion; so. somato- 

 pleure; sp. splanchnoplenre. 



Fig. 178. Optical sections of a Babbit's ovum at two stages closely 



FOLLOWING UPON THE SEGMENTATION. (After E. van Bcncden.) 



ep. epiblast; hij. primary hypoblast ; bp. Van Beneden's so-called blastopore. 



The shading of the epiblast and hypoblast is diagrammatic. 



B. E. II. IC 



