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



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pr.sf 



.^r-/jt. ij 



Pig. 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 tbe 

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

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

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

 amn. amnion; a.op.a.resL opaca; a.pel. area pellucida ; Up. blastopore; ei)i6. s. embryonic 

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

 medullary folds ; prot, v, pro to vertebrae ; pv. st. primitive knot. (After S. F. Clarke.) 



elliptical thickened patch — the embryonic 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 



