FURTHEK STUDIES OX THE GERMIXAL LAYERS ETC. 43 



the mass of mesoblast cells (darkly shaded in the figure) are 

 stretching outwards. This section again reminds us of the section 

 of Amphioxus shown in Fig. 40. 



I wish to call attention next to the embryo represented in Fig. 2. 

 This is interesting, because the gastral mesoblast in extending itself 

 forwards from the peristomal mesol^last has not yet reached the anterior 

 end r)f the embryonic shield, where there is as yet no head-fold, and we 

 are thus able to see in what manner this forward extension takes 

 place. Figs. 13-22 are sections of this embryo. If we look at Fig. 16 

 from the anterior part, we at once recognize its similarity to Fig. 12. 

 There is the epithelium stretching between the chorda- anläge and the 

 gut- hypoblast, and representing the walls of the- gut diverticulum. 

 From the outer end of this epithelium there stretches outwards the meso- 

 blastic mass (darkly shaded). We see here as well as in Fig. 12 a very 

 clear indication, (1) that the mesoblastic mass is divided into two 

 layers : the somatopleuric and the splanchnopleuric, and (2) that the 

 former is continuous with the above mentioned epithelium and the 

 latter with the gut-hypoblast. The latter fact necessitates the con- 

 clusion that the gut diverticulum in extending itself into the pro- 

 liferated mesoblast mass does so from its outer end. 



When the chorda, the mesoblast, and the gut-liypoblast become 

 independent of one another, at what point does the separation take 

 place? In tracing the series backwards, wepas;s suddenly I'rom sections 

 hke Fig. 16 to others like Fig. 17. This point corresponds in the 

 ventral view (Fig. 2) probably with the place where the two converging- 

 depressions come together and form the single median groove. The 

 change is brought about by the gut-hypoblast extending itself close to 

 the chorda- anläge, thus shutting off the gut diverticulum from the 

 enteric cavity. In other words, the point marked * in Fig. 16 has 

 moved itself to the point marked f . In Fig. 17 the splanchnic 



