242 



ESSENTIALS OF ZOOLOGY 



of the yolk, and the two layers are accompanied by the inter- 

 vening mesoderm. As has already been plain, the mesoderm 

 extends outwards from the embryo on each side and pos- 

 teriorly, and it has been resolved also into somatopleure and 

 splanchnopleure. In front of the head the mesoderm is absent. 

 The mesoderm-free region, however, is gradually narrowed by 

 the mesoderm increasing in a fold from each side. During the 

 second day, when the head is being formed and is being bent 

 downwards, a depression forms below the head, and in asso- 

 ciation therewith a new fold is produced in front. 



/.&. 



Ect. End. Am. \ m.b 



h.b. 



End. Notochord 



Dors, lip 



Midb. 



\ Lung Not. 

 f.b. St. I Am. 

 Thy.\ 

 Ht. 



Ect. 



Tal 



FIG. 124. Diagrammatic longitudinal sections of second- and third-day fowls 

 to illustrate the development of the amnion and the allantois. 



f.b., m.b., h.b., = fore-, mid-, and hind-brain ; h. = hypophysis ; St. = Stomo- 

 deum; Thy. = thyroid; Am. = amnion; All. = allantois. 



Amnion. This fold is the rudiment of the amnion. It con- 

 sists of ectoderm and endoderm. But as it forms, the mesoderm 

 wing on each side invades the region, and the two layers, united 

 at the edge, rapidly separate the endoderm from the ectoderm. 

 This is followed by an expansion of the body cavity, which 

 has the effect of removing the endoderm of the primitive fold 

 to a ventral position. The ectodermal fold, together with the 

 somatopleure mesoderm, is continued backwards as the embryo 

 sinks into the yolk. With the formation of the tail fold a 

 similar amnion fold appears around the hinder end of the 

 embryo. These folds rise and fuse as they advance along the 

 sides of the embryo. The process makes great progress in the 



