352 



ZOOLOGY 



SECT. 



equivalent of the mass of yolk-cells of the Frog, and the shallow 

 space between it and the yolk represents the segmentation-cavity. 

 As the blastoderm extends (Fig. 1017), it becomes distinguishable 

 into a central clearer area area pellucida (a. pel.) and a peripheral 



FIG. 1017. A D, early stages in the development of the Alligator. A, stage with em- 

 bryonic shield, primitive knot and blastopore ; E, considerably later stage in which the 

 medullary groove has become formed, together with the head-fold of the embryo and the 

 head-fold of the amnion ; C, somewhat later stage with well-developed medullary folds and 

 medullary groove ; D, later stage in which the medullary groove has become partly closed 

 in by the medullary folds and in which six pairs of protovertebrse have become developed. 

 amn. amnion ; a. op. area opaca ; a. pel. area pellucida ; blp. blastopore ; emb. s. embryonic 

 shield ; /. br. fore-brain ; h.br. hind-brain ; M./. head-fold ; m.br. mid-brain ; med.f. 

 medullary folds ; prot. v. protovertebrae ; pr. st. primitive knot. (After S. F. Clarke.) 



whitish zone area opaca (a. op.). On the former now appears an 

 elliptical thickened patch the embryonic shield (emb. s.) which is 

 formed by the ectoderm cells in this region assuming a cylindrical 

 form while remaining flat elsewhere. Behind the embryonic 

 shield appears a thickening, due to a proliferation of the ectoderm 



