XITI 



PHYLUM CHORDATA 



3G3 



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

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

 into a central clearer area — areapellueida (a. pel.) — and a peripheral 

 whitish zone — ai-ea opaca (a. op.). On the former now appears an 



a.ptl 



ti pt I 



emb.: 



pr.st. 



fjrs-t 



.fjral. i 



Fig. 1006. — A — B, 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 protovertebrae have become developed. 

 amn. amnion; a.oy). area opaca; n.pel. area pellucida ; bl/i. blastopore; «)i/j. .s. embryonic 

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

 medullary folds ; 'pi'ot. v. protovertebia; ; pr.st. primitive knot. (After S. F. Clarke.) 



elliptical thickened patch — the emhryonic shield (emh. 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 



