442 . Leland Griggs. 



Stage 7. The first neuromere folds down to form the infundib- 

 ular depression. 



Stage 8. The neural crests move together and fuse to form 

 the neural tube. 



Stage 9. The neural crests become segmented. The otic 

 pit appears. 



Description of Embryos 



Stage 1 (Fig. 1, B). The posterior germinal depression 

 (pgd fig. 1, B), the first structure to appear on the surface of the 

 egg in front of the blastopore, is formed while the egg is still 

 spherical before the yolk plug has been withdrawn. There are 

 no indications of any growth in length of the depression, or of 

 any origin at a constant point from which it grows either anteri- 

 orly or posteriorly, but the groove appears at the very outset 

 as a long shallow depression of uniform length in different eggs. 



In order to distinguish between this groove and the grooves 

 which appear later it is important to note carefully its character. 

 It extends from a point an appreciable distance in front of the 

 blastopore to a point about half the distance across the upper 

 hemisphere of the egg. It is nearly as wide as the diameter of 

 the blastopore just before the latter changes from its circular 

 to its oval form. Its depth varies owing to the presence of 

 several shallow pits one of which is usually found at the anterior 

 end forming a fairly well marked anterior limit to the depression. 



It has already been mentioned that there is considerable dif- 

 ference of opinion in regard to the number of grooves which 

 appear on the surface of the amphibian egg. Miss Johnson 

 ('84), Schultze ('88) and Erlanger ('90) have maintained that 

 there is but one groove, the primitive groove, formed by a con- 

 cresence of the lips of the blastopore and then extending forward 

 over the surface of the embryo. A study of these early stages 

 of Amblystoma supports the position taken by the later authors, 

 Robinson and Assheton ('91), Jordan ('93), and Eycleshymer 

 ('95), that there are two grooves, the posterior one derived from 

 the blastopore showing a fusion of cell layers and the anterior 



