378 EYCLESHYMER. [Vol. X. 



et fusionnees. Le sillon median est done comparable a la for- 

 mation designee par Hatschek sous le nom de raph6 gastru- 

 laire (Gastrularaphe) des Ascidies, de I'Amphioxus et des 

 Annelides." 



These different views manifestly rest upon different con- 

 ceptions of the extent of the blastopore and its method of 

 closure. 



A second source of confusion lies in the fact that the primi- 

 tive, neural, and dorsal grooves are considered as one structure. 

 In Amblystoma and Rana palustris there is a primitive groove, 

 but this groove is not in front or behind the blastopore. It is the 

 blastopore, or its remnant. In section, the line representing the 

 area along which the lips have fused, is always present beneath 

 it. A second groove is also often present, extending forward 

 from the dorsal lip of the blastopore. It may appear as a 

 slight depression just in front of the dorsal lip, extending in 

 either direction, uniting the primitive and neural grooves, or it 

 may be entirely absent. Sections through this groove show no 

 line of fusion, nor in any way suggest that such has occurred. 



The one fact which I wish to emphasize, is that this groove 

 is in no way indicative of the extent of the primitive streak, 

 but is a part of the neural groove, having arisen in precisely 

 the same manner, while the primitive groove has an entirely 

 different origin. 



6. The Epiblast and Mesoblast in Petromyzon. 



A glance at the literature on this form shows a wide dispar- 

 ity of views concerning the process of mesoblast formation and 

 extension. This, together with certain facts concerning the 

 thinning of the roof of the segmentation cavity, preceding gas- 

 trulation, have led me to record a part of my observations. 



In a late cleavage stage the roof of the cavity is composed 

 of from two to three layers of cells, resembling the condition 

 already noticed in Amblystoma; just before gastrulation the 

 roof thins to a single layer of cells (PI. XXI, Fig. 5), as 

 stated by Schultze ('56), Scott ('81), and Shipley ('88). Kupffer 

 ('90) and Hatta ('92), have likewise observed the thinning, but 

 hold that the single layer is not reached. 



