EMBRYOLOGY OF "CRYPTOBRANCHUS 497 



thin. As stated by Ishikawa, the polygonal figures formed on 

 the surface of the blastocoele-segment (fenestra) are perhaps due 

 to the pressure which produces the gradual diminution of its 

 area; but the cells of the fenestra are not compacted together to 

 any considerable extent, since the gastrocoele roof and wall merely 

 grow under them. 



In view of the later history, it is evident even from surface 

 views that in the stage shown in figures 116 and 123 the forma- 

 tive material for the embryo is mainly concentrated in the equa- 

 torial region as a broad band or zone of cells, wider in its poste- 

 rior portion. As will be shown in the description of the internal 

 structure, this equatorial zone as distinguished in surface views 

 is only roughly comparable to the germ ring of fishes. 



My material is lacking for the study of the early gastrula 

 stages of Necturus; late gastrula stages differ from Cryptobran- 

 chus chiefly in that the blastopore earlier becomes a complete 

 circle. In Spelerpes, according to Goodale ('11), no ventral lip 

 is formed to the blastopore. As compared with urodele and 

 anuran eggs in general, the blastopore of Cryptobranchus is late 

 in closing; in its mode of gastrulation the egg of Cryptobranchus 

 approaches more nearly the type observed in meroblastic eggs. 



Stage 12: {figs. 138 to 150 and 223 to 225). This stage is char- 

 acterized by the presence of the neural groove and is terminated 

 by the appearance of the neural folds. The neural groove appears 

 about three days after the beginning of gastrulation. 



At the time of the earliest indications of the neural groove, 

 the blastopore has just become a complete circle. At the close 

 of the preceding stage it had a diameter of about 94 degrees; 

 it now rapidly becomes smaller, so that before the appearance 

 of the neural folds its diameter averages about 26 degrees (figs. 

 138 to 145). 



During the early part of this stage the yolk plug is characterized 

 by a broad crescent-shaped or horseshoe-shaped area of smaller 

 cells lying ventrad and laterad to the macromeres (figs. 138 and 

 139) . Along the lateral line of transition between the macromeres 

 and these smaller cells, the cells appear compressed and exhibit 

 a tendency to line up and merge their cleavage furrows (figs. 



