FURTHER STUDIES OX THE GERMINAL LAYERS ETC. 47 



the concavity turned backwards. 



Of an older embryo (Figs. 4 and 4a) with the head-fold and the 

 commencing amnion, a section from the head (Fig. 23) has already been 

 sufficiently described. As we pass l^nckward, we see that the cavity of the 

 giit-diverticuhim is obliterated (Fig, 24). A little further behind, three 

 structures ; the chovda-anlage, the niesoblast, and the gut or definitive 

 hypoblast become separated after the manner shown in Figs. 83-86. 

 We then obtain a section such as Fig. 25. In the middle region of the 

 body, the formation of the cliorda dorsnli.s is completed (Figs. 26 and 

 2 7) exactly in tlie way described in our T r i o n y x paper (No. 1 ). Further 

 backwards, the definitive hypoblsist of two sides whicli is united 

 across in Fig. 27, separates again from each other (Fig. 28) and still 

 more posteriorly, the three structures nbo^e mentioned become united 

 once înore (Figs. 29 and 30). Figs. 31 and 32 are so ex;)ctly like 

 Figs. 21 and 22, or similar figures in Trionyx, that they need no 

 special description here. 



Trionyx Japonicus. Schiegei 



After making (jut the earlier phases in the formation of the gastral 

 mesoblast in Clemmys, I thought it necessary to reexamine Trio- 

 n y X. I succeeded in finding two embryos in which the gastral mesoblast 

 had not yet reached the anterior end and was in process of formation. 

 After cutting them into sections, I fjund that they ])resent exactlv the 

 same structures as the Clem my s embryo given in F'ig. 2. In order to 

 prove this point, [ have selected and figured two sections (F'igs. 37 and 

 38) from one uf the embryos. In Fig, 87, tlie gut-hypoblast is at 

 some distance from the median line, and there is a stretch of epithelium 

 between it and the (.'hovd-d-aJilage, which is continuous externalJv with 

 the proliferated mesoblast mass. Several sections behind, in F^ig. S8, 

 the gut-hypoblast has shifted its inner end to the chovdn-anlagr, and 



