248 The Alligator and Its Allies 



determined because of a break in the series at this 

 point. The section passes through the posterior 

 limit of the head-fold. The medullary groove 

 (mg) is very deep and comparatively wide ; around 

 its sides the germ layers are so closely associated 

 that they may scarcely be distinguished. Ventral 

 to the medullary groove the foregut (/g) is seen 

 as a crescentic slit. 



Figure yd is about a dozen sections posterior to 

 the one just described and is about one seventh 

 the length of the embr3^o from the anterior end. 

 The embryo is much more compressed, in a dorso- 

 ventral direction, and the medullary groove (mg) 

 is correspondingly more shallow. Where the ecto- 

 derm (ec) curves over to form the medullary folds 

 it becomes much more compact and somewhat 

 thicker. The notochord (lit) is large and distinct, 

 but is still fused with the entoderm {e7i). The 

 mesoderm (mes) forms a well-defined layer, en- 

 tirely distinct from both the notochord and the 

 entoderm. From this region, as we pass caudad, 

 the size of the embryo in cross-section gradually 

 decreases and the medullary groove becomes more 

 shallow. 



Figure ye is about one third of the length of the 

 embryo from the posterior end, and is only a few 

 sections from the caudal end of the medullary 

 groove. The ectoderm (ec) is much thinner than 

 in the preceding figure and the medullary groove 

 (mg) is much more shallow. The notochord (iit) 



f! 



