FURTHER STUDIES ON THE GERMINAL LAYERS ETC. 41. 



proceeding further, from observations which in my opinion make the 

 above conclusion inevitable. 



I wish to call attention first to Fig. 23 (PJ. I\\), from the head 

 region of a somewhat old embryo given in Figs. 4 and 4a. Here the 

 gut-hypoblast (or Darm-E)itohlast), instead of juiuing the chorda- 

 hypoblast, turns upwards and becomes continuous with the meso- 

 blastic mass. The chorda-hypoblast in immediate continuation of 

 the chorda also turns upwards and becomes merged in the meso- 

 blastic mass. Between the chorda-, and gut-, hypoblast, a diver- 

 ticulum of the archenteron enters into the mesoblastic mass on each 

 side of the chorda. Now it seems to me that this section can have but 

 one interpretation, and represents, as clearly as we can reasonably 

 expect in a meroblastic egg, the process of the mesoblast development 

 so well known in Amphioxus. Here, however, from the outer and 

 upper walls of the diverticulum (I am speaking more especially of the 

 left side of the section), a mass of cells stretches outward. That this 

 mass has budded out from the walls of the diverticulum is evident 

 from the karyokinetic figures which we see in it. This section ought 

 therefore, strictly speaking, to be compared to such a section of Am- 

 phioxus as is given in Fig. 40 (PI. TV.), in which the mesoblast has 

 stretched more ventrally. To make the C(3mparison yet closer, the 

 gut would have to be still in open comnuuiication with the coelom 

 in the section of Amphioxus. Now, when the chorda, the meso- 

 blast and the gut-hypoblast finally separate from one another in 

 Clem my s , at what point does this se})aration take place ? In order 

 to answer this question conclusively, I have introduced Figs. 33-36 

 from the head region of another embryo of about the same stage. 

 Fig. 33 is essentially like Fig. 23. The right side of this secti(jn is 

 more forward than the left side and is significant, for the gut-diver- 

 ticulum is quite simple, and there has not yet been as much pro- 



