FORMATION OF GERMINAL LAYERS IN CHELONIA. 43 



Hert wig's fig. 9, Taf. ii, is the frontal section through the 

 line c — d of fig. 4, Taf. ii. It is substantially the same as our 

 fig. 9, although there is a closer resemblance between it and our 

 fig. 18, as we have already shown. 



Unfortunately Hertwig does not give a cross section of the 

 front region of an embrvo which has not yet developed the 

 mesoblast; but we are sure it will be essentially like our 

 figs. 13 and 14, although we cannot expect to find the lateral 

 parts composed of a network of cells. 



Now, as to the originofthemesoblast, our results agree with 

 Hertwig's account as completely as could be desired. In the 

 region behind the blastopore he says the mesoblast arises as an 

 unpaired mass in the Amphibia. Such is the case with 

 Trionyx, as shown in our figs. 7, 8,16, and 24. In front of the 

 blastopore the mesoblast arises as paired masses separated from 

 each other in the median line by the chorda-entoblast. For 

 this point compare our figs. 17 and 20, or, best of all, figs. 25 

 and 29, with Hertwig's figs. 1 and 2 (Taf. iii) of Triton. In 

 the latter the chorda-entoblast passes into the parietal layer 

 of the mesoblast, while the darm-entoblast is reflected just 

 where it abuts against the chorda-entoblast, and passes into 

 the visceral layer of the mesoblast, thus constituting what 

 amounts to a pair of diverticula from the alimentary canal, one 

 on each side of the chorda, repeating what is seen in Amphioxus. 

 Hertwig has marked the entrance to these rudimentary diverti- 

 cula with a star (*) in his figures. We have also marked in 

 our figures what we consider to be the corresponding spots 

 with the same mark (■^). We think that morphologists will 

 not find any difiiculty in recognising in Trionyx tlie relations 

 closely similar to those in Amphibia. In Trionyx the meso- 

 blastic mass becomes continuous on each side with the ento- 

 blast, just at the point where the chorda- and the darm- 

 entoblast meet each other. The cells being much smaller in 

 Trionyx than in Triton, it is not possible to distinguish the 

 parietal from the visceral layer of the mesoblast ; but if both 

 the chorda- and the darm-entoblast pass into the mesoblastic 

 mass, the relations found here amount to the same thing as 



