42 K. MrrstTKURT. 



liferation of raesoblast cells from its walls as on tlie left side or in 

 Fig. 23. But the point that I want to be particularly noticed in the 

 section is on the left side which, as I said before, is essentially like 

 the corresponding' side of Fig. 23. Two sections behind this (Fig. 34), 

 the hypoblast has pushed towards the median line and obliterated 

 the gut diverticulum, of which, however, its outer and inner walls 

 are as yet distinguishable. Fig. 35 is two sections still further 

 behind, where the parts are pressed closely together. Two or three 

 sections yet further back, the cliorda, the gut-hypoblast, and the 

 mesoblast have entirely separated from one .another (Fig. 36). 

 This series shows conclusively that the part of the primitive 

 hypoblast immediately adjoining the chorda becomes 

 a part of the mesoblast. 



Kow turn to Fig. 12 (PI. Ill), taken from the head region of the 

 embryo shown in Figs. 8 and da, somewhat younger than that of Fig. 

 4, from which Fig. 23 is taken. In this section, l)etween the chorda- 

 anlage and the gut-hypoblast, there is a stretch of columnar epithelium 

 slightly arched upwards. From the point where this epithelium joins 

 the gut-hypoblast, there stretches outwards a mass of mesoblast cells 

 between the epiblast and hypol)last. Now there can be no reasonable 

 doubt that this section presents substantially the same structure as the 

 left side of Fig. 23. If the epiblast did not press down so closely, and 

 if the epithelium (lightly shaded in the figure) between the points 

 marked with f and * arched upwards with a greater curvature, the 

 section would be exactly like the left side of Fig. 23. We therefore 

 come to the conclusion, that the epithelium (lightly shaded in Fig. 12), 

 stretching between the chorda- anläge and the gut-hypoblast, and con- 

 tinuous with both, ought to be considered as the walls of the gut- 

 diverticulum, and "a slight l)ay l)etween f and * as representing the 

 gut-diverticulum itself. Fi-om the outer walls of this diverticulum, 



