XIII 



PHYLUM CHORDATA 



363 



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

 As the blastoderm extends (Fig. 1006) it becomes distinguishable 

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

 whitish zone area o-paca (a. op.). On the former now appears an 



Fio. 1006. A D, early stages in the development of the Alligator. A, stage with em- 

 bryonic shield, primitive knot and blastopore ; B, 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 ; Up. blastopore ; emb. s. embryonic 

 shield; f.br. fore-brain; h.br, hind-brain; hd.f. head-fold; rn. br. mid-brain; med. 

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



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 J elsewhere. Behind the embryonic 

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



