The Development of the AlHgator 335 



unopened egg is due chiefly to stains produced by 

 the decayed vegetation of the nest. At hatching 

 the young alHgator 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 

 development 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 distinction can be made between the 

 mesoderm derived from the ectoderm and that 

 derived from the entoderm. 



The ectoderm shows during the earUer stages a 

 very great increase in thickness along the median 

 longitudinal axis of the embryo. 



The notochord is apparently of entodermal ori- 

 gin, though in the posterior regions, where the 

 germ layers are continuous with each other, it is 

 difficult to decide with certainty. 



