ç)50 K. MITSUKURI. 



extends itself gradually forward«, pan passu with the 

 anterior enlargement of the Primitive Knol). 



4. The roof of thi« invagination-ca^■ity which be- 

 comes continuous with the epiblast of the embryonic 

 shield at the anterior lip of the blastopore, assumes a 

 colunmar arrangement, the process beginning at the 

 posterior end and proceeding gradually forwards. Out of 

 the median part of it is established the Chorda dorsalis, 

 and from a certain stretch of columnar epithelium on each 

 side of it is developed the gastral mesoblast. 



5. The floor of the invagination-c;ivity is divided 

 into two parts: — ((/) the posterior which proliferates the 

 peristomal mesoblast, and (h) the anterior which losing 

 deiiniteness is finally absorbed, together with the whole 

 thickness of the cell network placed beneath it, ih ns putting 

 tlie invaginaiion-cacit ij in commun ication with the large suIj- 

 germinal cacittj in ttte yolk. 



6. The primitive knob which was gradually spread- 

 ing itself over the ventral surfjice of the embryonic shield 

 before the breaking through of the in vagination-cavity 

 continues to do so after that event. It spreads from the 

 spot where the in vagination -ca vity first broke through 

 away towards the periphery of the shield. Its advance 

 in later stages is marked by a zone of cell -net work 

 with a compact central area. AVhen the whole of the 

 ventral surface of the embryonic shield has been 

 covered, the process sto])s. The cell-network afterwards 

 changes into compact cellular sheets. 



7. The head-fold is formed some distance behind 

 the anterior edge of the embryonic shield. 



