DEVELOPMENT OE THE AMERICAN ALLIGATOR — REESE 6l 



that purpose. The blotchy appearance of the unopened egg is due 

 chiefly to stains produced by the decayed vegetation of the nest. At 

 hatching the young alligator is about 20 cm. long, nearly three times 

 the length of the egg ; but the tail is so compressed that, though it 

 makes up about half of the length of the animal, it takes up very 

 little room in the egg. 



Summary 



Owing to the fact that the embryo may undergo considerable 

 development before the egg is laid, and also to the unusual difficulty 

 of removing the very young embryos, the earlier stages of develop- 

 ment are very difficult to obtain. 



The mesoderm seems to be derived chiefly by proliferation from 

 the entoderm, in which way all of that anterior to the blastopore 

 arises. Posterior to the blastopore the mesoderm is proliferated 

 from the lower side of the ectoderm in the usual way. No distinc- 

 tion can be made between the mesoderm derived from the ectoderm 

 and that derived from the entoderm. 



The ectoderm shows during the earlier stages a very great in- 

 crease in thickness along the median longitudinal axis of the embryo. 



The notochord is apparently of entodermal origin, though in the 

 posterior regions, where the germ layers are continuous with each 

 other, it is difficult to decide with certainty. 



The medullary folds have a curious origin, difficult to explain 

 without the use of figures. They are continuous posteriorly with 

 the primitive streak, so that it is impossible to tell where the medul- 

 lary groove ends and the primitive groove begins, unless the dorsal 

 opening of the blastopore be taken as the dividing point. 



The amnion develops rapidly, and entirely from the anterior end. 



The blastopore or neurenteric canal is a very distinct feature of 

 all the earlier stages up to about the time of closure of the medullary 

 canal. 



Preceding the ordinary cranial flexure there is a sort of temporary 

 bending of the head region, clue apparently to the formation of the 

 head-fold. 



During the earlier stages of development the anterior end of the 

 embryo is pushed under the surface of the blastoderm, and is hence 

 not seen from above. 



Body torsion is not so definite in direction as in the chick, some 

 embryos lying on the right side, others on the left. 



Of the gill clefts, three clearly open to the exterior and probably 

 a fourth also. A probable fifth cleft was seen in sections and in one 

 surface view. 



