246 K- MITSUKURI. 



thnt ill the anterior part of the embryonic shield, the dorsal roof of 

 the archenteric cavity divides itself into two layers : (1) a lower c^ne 

 consisting of a single layer of low cells representing the definitive 

 hypoblast, and (2) an upper one consisting of stellate branched cells 

 representing the " Rumpf-mesoblast " (his Figs. 22 & 23). In the 

 course of this separation, the dorsal roof which is nt first composed 

 of compact columnar cells becomes permeated by vacuoles, aiid he 

 says tliat " das im Flächenbilde eruirte Xetz der Ausdruck fiu' die aus 

 dem Verbände des Urdarm-epithelhofes (sel. oberer Ürdai-mwand) 

 losgelösten Mesodermstränge war, welche sieh im Furcluingsspalte 

 centrifua-al zwischen Ektoderm und Paraderm weiter vorschieben " 

 (]^. 434). He thus calls the ai'e:i of the network with the central 

 compact ])nrt the '' Rumpfmesodermhof." Moreover he makes thi.s 

 [)r<jcess of the mesoblast formation begin at the cnminl end and proceed 

 backwards. He also says thnt "'die periphere Ausbreitung des Meso- 

 dermhofes nicht im proportionalen Verhältnisse znr Grösse der Area 

 embryonnlis (sei. Euibryoniilschild) steht " (p. 434j. Mehnert's views 

 C!in not be reconciled with mine: one of us is wrong. Except as to 

 the single ])oint that the network grows centrifugally, I am obliged to 

 differ from him in almost every particular. This network has in my 

 opinion nothing to do with the process of the mesoblast formation. 

 My views on the latter process have already been given in great detail 

 in two former papers (Contrib. I. & III.) and I do not intend to 

 go into them again in this paper. The network is simply the surface 

 expression of cells from the primitive knob spreading themselves over 

 the ventral fnce of the embryonic shield. In what light we ought to 

 regard this pi-ocess I shall discuss farther on. But whatever it is, it 

 does not begin at the cranial end and proceed backwards. In obt:iin- 

 ing materials for the ])resent investigation, I opened on consecutive 

 days a certain number of eggs from tlie same deposits, and observed 



