270 ^- MITSUKUEI. 



ganzen Processes natürlich nicht melir erschliessen, oh es sich nm 

 echte Divertikelbildiino- oder nm Unterwuclisung' von Seiten der 

 Urdarnifiilten handelt, oh das Divertikel das l^rimare nnd die solide 

 Mesoblastmasse das Secundäre ist, oder uintrekehrt." Now, the fact is 

 familiar to every embryologist that at a given stage of development a 

 structure, one part of which is already finished may show at another 

 ])(3rtion of its length only the commencing phases of the process of fn-ma- 

 tion, so that one can see in one and the same specimen the whole ])rocess 

 from the beginning to the end. Such seems to me to be the case with 

 the mesoblast in the CI em my s embryo from which my figure 23 is 

 taken. The fact tliat the mesoblast formation is complete and the 

 chorda is cut off in the middle dorsal region, does not necessarily vitiate 

 what is seen in the head region : here the ])rocess of the mesoblast 

 formation is in a less advanced phase, and if a diverticulum is seen 

 there, it is highly probable that a diverticulum is a feature of the 

 mesoblast formation. That there is no such distinct diverticulum seen 

 earlier in the middle dorsal region is because the epi blast presses closely 

 down, and there is no space for the diverticula to curve upwards to 

 any large extent as in the head region. I think, I have sufficiently 

 demonstrated in my Contrib. Ill, that the diverticulum in the head 

 region corresponds to that part of the primitive hypoblast in the dorsal 

 region which Will calls the " Zwischenplatte," and that this 7nust 

 therefore be regarded as a shallow diverticulum. 



Will also objects to my views on the following grounds : " P)ei 

 der Auffiissungder Zwischenplatte als ein gestrecktes Divertikel miisste 

 der solide Teil des gastralen Mesoderms (war. in Fig. 1 ]>.) nicht an 

 dem Rande der Z\vischen])latte inserirt, s(jndern aus der Mitte dei- 

 letzteren hervorgewuchert sein." (Anat. Anz. No. 20, l<Si)3. p. 6<S]) 

 Again "Wäre die Zwisclieni)latte ein abgeflachtes Mesodermdivertikel, 

 •so musste aus iVir sowohl der somatische wie die splanchnische Mesobkist 



