232 K- MITSUKURI. 



there is a second much smaller, irregularly circular white patch. 

 This is the structure called the "Primitivplatte" (the primitive 

 plate) by Will* (No. 1<S); and in later stages, when cells added on from 

 the subjacent yolk form an accumulation, is the " P r i m i t i v k n o t e n " 

 (the primitive knob) of Mehnert (No. <S). These names will be adopted 

 and used interchangeably in this paper, for it is after all difficult fo 

 distinguish when the state of the plate ends and that of the knob 

 begins. One stage of it is also called the • ' S i c h e 1 " by Will (No. 1 8). 

 In the dorsal view (Fig. 1) there is already in the middle of this area 

 a large, transversely elongated opening leading into a spacious cavity, 

 For convenience in description, I anticipate my conclusions by stating 

 here that I consider this cavity to be the archenteron and its 

 dorsal opening to be the blastopore. In the ventral view (Fig. la), 

 the appropriateness of calling the above area a " knob " is clearly 

 seen, for it is a thick accumnuilation of cells projecting much more 

 than the adjacent parts into the yolk. It is important above all to 

 notice that the archenteron at this stage is not open on the 

 ventral side, although it can be seen from that side through the 

 cell-mass in the specimen figured. 



Fig. 9, PI. VIII. is a median longitudinal section through another 

 embryo of the same stage as that represented in Figs. 1 and la. In 

 this the yolk was left intact on the ventral face, so that it represents 

 no doubt a more nearly normal state of things than if the yolk had 

 been removed as in Fig. la. The blastoderm has already extended 

 itself over a wide area and, with the exception of the primitive 

 plate, is divided throughout into two layers : (a) the superficial layer 

 (the e pi blast of authors, the blast ophor of Van Beneden), and 



* Also in No. 21. 1 regret that this paper came to my hauds only after the present 

 contriljutiou was nearly in shape to be given to the printer. I could not therefore make as 

 much use of it as I should have liked to do. 



