CRANGON VULGARIS. Ill 



I have fully satisfied myself as to the limits of the origin 

 of the mesoderm. It certainly arises from both sides and 

 from the anterior margin of the blastopore ; whether it 

 also has its origin from the posterior margin or not, I can- 

 not positively say, though I am inclined to think that it 

 does not. It certainly does not form there as abundantly 

 as it does in front. Later, the mesoderm may be recog- 

 nized by the fusiform cells with small nuclei crowded be- 

 tween the ectodermal structures and the yolk. It acquires 

 its greatest development at first in the abdomen but ap- 

 pears only as a thin sheet in the cephalothoracic region 

 until the embryo is nearly ready to hatch. I have at no 

 time seen anything looking like 'mesenchyme ' nor have I 

 seen anything that could be interpreted as a budding of 

 mesoderm cells from either ectoderm or eudoderm. Nei- 

 ther do I see anyway, looking at Crangon alone, of decid- 

 ing from which of the other germinal layers the mesoderm 

 arises. It seems to come from the junction of the two. 



Before the next stage becomes outlined the blastopore 

 becomes completely closed. As a considerable time elapses 

 between this closure and the formation of the stomodeal 

 and proctodeal invagination it is a matter of considerable 

 difficulty to say exactly what are the relations of the blas- 

 topore to either mouth or anus. As no appendages are as 

 yet developed, there are no landmarks by which the posi- 

 tion of the blastopore can be recognized in surface views 

 and all that there is to guide one is the general outline of 

 the rapidly changing germinal area. From this it would 

 appear as if the anus arose either within or a very little 

 in front of the position formerly occupied by the mouth 

 of the gastrula ; and I am inclined to the former view, 

 since there is in the meantime a very rapid division and 

 hence considerable extension of the circumblastoporal 

 cells. 



