244 ^- MITSUKÜEI. 



low (Fig«. 4rt and 16). The part that has been covered by the cells 

 of the primitive knob can be very plainly distinguished on u ventral 

 view of the blastoderm, showing mostly a trabecular network (Fig. 

 Aa). This gradual spreading of the cells from the primitive knob 

 (jver the ventral surface of the embryonic shield is continued long 

 after the breaking through of the invagination-cavity. Figs. 5 and 

 ba are only a little advanced on Figs. 4 and 4a. The area of the 

 network is not yet very large, ])ut in the stage next introduced 

 (Figs. () and 6a) it has expanded itself considerably. A great change 

 is now noticeable : a, circidar area at its centre shows no longer a 

 network but presents a smooth compact surface. This is produced by 

 a continuation forwards of the process by which cells in the roof of the 

 archenteric-cavity, beginning at the dorsal lip of the blastopore, have 

 gradual 1\- assumed the columnar shape and formed themselves into a 

 compact sheet (Figs. 13-18) 



In Figs. 7 and 7a the spreading of the part derived from the 

 primitive knob has gone one step further. Not only is the area 

 occupied by the network larger but the compact part in the centre is 

 considerably enlarged by its extension anteriorly in the median line. 

 There is also anotlier noteworthy new feature : at tlie anterior end of 

 the median compact area, there is a slight transverse ridge. This is 

 the com m e n c i n g head- f o Id. The last stage in which I was able 

 to dete(.'t traces of the network is shown in Figs. 8 and 8a. At the 

 front end of the embryonic shield, a patch of the network could Ije 

 faintly traced. I tliink it almost certain that the part derived from 

 the j)riniitive knob does not extend itself much beyond the area of the 

 embryonic shield, and that it gradually thins itself out and ends by 

 becoudng simply continuous with the primitive lower la^er at or near 

 the periphery of this area (Figs. 17 & 18). Witli the exception of 

 the patch above-mentioned, the ventral surface of tlie embryonic shield 



