ox THE (iAöTRULATION IX I'KTROMYZON. OO 



remains as the extrnial c<>at of the ovum merits the name of the 

 ectoderm. 



Having described above the main course of the gastrulation, 

 I may be permitted to give some more considerations on a few 

 points : 



1. I have stated before that I have come to consider the 

 stage given in Fig. 1 as an old morula stage, and not as a 

 blastula. This is due to the following consideration. It seems 

 to me correct that under the term " blastula " is understood 

 a stage in which blastomeres are no longer in a merely aggregated 

 state, but are converted into the form of an epithelium, since 

 this histologically differentiated condition ought certainly to be 

 distinguished from the stages that consist (jf indifferent cells 

 only, as is very obvious in Ami^hioxus. Applying this view to 

 the present case, the stage given in Fig. 1 is not really gone 

 further than an old morula. In this case the blastulation, as 

 indicated by differentiation of the blastomeres into an epithelium, 

 should be looked upon as being much delayed ; it is still being- 

 carried on during the whole period of the gastrulation and is 

 finished only a little earlier than the latter process. In other 

 words, the two processess, blastulation and gastrulation, overlap 

 each other to a great extent in the period of their occurrence. 

 The prime cause of this belated mode of development is in- 

 disputably due to delay of segmentation on account of an en(n"mous 

 accumulation of yolk within the ovum. 



2. I have n<jt been able to detect in any stage what might 

 be considered as " concrescence." It is trae that at a much 

 later stage when the embryo begins to grow in its long axis, 



