DEVELOPMENT OF GALLUS 



205 



embryo in the chick will develop from only a part of the egg 

 and blastoderm. The remainder will give rise to the 

 membranes outside the embryo. At first, all that exists of 

 the embryo proper is the middle line of its back represented 

 by the primitive streak. Its sides are formed as this streak- 



mb, m 



Fig. 93. — Gallus : embryo chick after 30 hours' incubation seen by reflected 

 light A from the dorsal, B from the ventral side. (After Jenkinson.) 



aip, anterior intestinal portal ; bi, blood-islands ; fb, forebrain ; hb, 

 hind brain ; ht, heart ; mb, midbrain. Other letters as Figs. 90 and 91. 



region rises and becomes folded up from the surface of the 

 blastoderm around it. This process begins in front with the 

 formation of the head-fold ; by this means the ectoderm, 

 notochord, mesoderm, and endoderm are lifted off from the 

 surface of the underlying yolk, and a cavity appears between 



