STUDIES IN MAMMALIAN EMBRYOLOGY. 527 



blasts Rabl) with the similar formations in Amphibia is easy 

 enough. 



And as for the so-called peristomal mesoblast, we shall have 

 to look for that in the first place in the immediate vicinity 

 along the sides and at the posterior end (Endwulst) of the 

 gastrula ridge. The identification of the lateral wings of meso- 

 blast that spring from the gastrula ridge with the Amphibian 

 peristomal mesoblast has already been eflPectuated by Rabl 

 himself. Still it appears to me that the woodcut which he 

 gives on p. 173 of his essay (24) is not complete, but ought to 

 show a posterior loop connecting the two parallel dotted lines, 

 thereby expressing that from the gastrula-ridge mesoblast does 

 not originate in the shape of two separated halves which after- 

 wards coalesce posteriorly, but that the plate of mesoblast 

 could better be compared to a fan which was brought to its 

 maximum of expansion (see above, p. 514). It is the exten- 

 sion backwards of this continuous mesoblast plate that can of 

 course be directly compared to what takes place in Amphibia. 

 For the shrew, I have above demonstrated that at the posterior 

 end of the gastrula ridge new cells are added to this plate of 

 mesoblast, which directly spring from the underlying hypo- 

 blast belonging to the modified annular zone. This pheno- 

 menon is again comparable to what Avas noticed in Amphibia 

 concerning the participation of a certain number of yolk-cells 

 towards the formation of the peristomal mesoblast. I have 

 also noticed above that laterally numerous indications were 

 found of the actual participation of hypoblast-cells to a 

 similar end ; whereas for the sheep. Bonnet contends that to 

 a no less considerable extent the hypoblast participates in 

 the formation of peripheral mesoblast along the whole ex- 

 tension of an annular region slightly larger than the embryonic 

 shield. To this annular zone, which I have above alluded 

 to more fully, he has given the name of " Mesoblast-hof." I 

 have above described an exactly similar ring of tissue in the 

 shrew, which is at all events peculiarly modified peripheral hypo- 

 blast, even if we cannot fix for the present the exact extent 

 to which either the whole or only a part of it actually produces 



