12,2 MCMURRICH. [Vol. XI. 



Reinhard ('8?) and Roule ('9i, '92, '92 a) have already been stated 

 (p. 96), and it is very clear that they receive not the slightest 

 confirmation from the forms I have studied, and are at variance 

 with the results of the other observers who have studied the 

 embryology of the Isopods. As regards the Amphipods much 

 has yet to be done before a proper idea of their early develop- 

 ment is obtained; the results of Dr. Sophie Pereyaslawzew 

 ('88) and her co-workers Mesdames Rossiiskaya - Koschewni- 

 kowa and Wagner, are evidently quite inadequate, the meso- 

 derm being stated to arise in connection with the limbs, 

 evidently not having been traced even approximately to its 

 origin. The results of Delia Valle ('93) though much more 

 carefully worked out, still leave much for later investigation, it 

 being impossible to harmonize them with what occurs in other 

 Crustacea. 



There are reasons then for doubting the correctness of the 

 views of those authors who ascribe an extra-blastoporic origin 

 for the mesendodermal elements in the Crustacea except in the 

 case of the Decapods, and since these are the most highly 

 specialized of all the Crustacea, the precocious formation of 

 vitellophags which they show may with justice be regarded as 

 a secondary phenomenon. It remains to consider what is to 

 be regarded as the blastopore in the Crustacea, and what the 

 significance of the various phenomena which have been de- 

 scribed in connection with it. 



One naturally turns to the simpler forms to get an idea as 

 to the primitive character of the blastopore, and in the Phyllo- 

 pods one is at once met by two striking facts, (i) the mesoderm 

 and endoderm have a common origin and cannot at the time of 

 their formation be distinguished from one another, and (2) they 

 arise by the immigration of certain cells into what would be 

 the blastocoel were the yolk not present, there being no indi- 

 cation of an invagination. These two facts are true of Daphnia 

 and DapJmella (Samassa, '93) and of Branchipus (Brauer, -92), 

 and according to Samassa's account of Moina also, though 

 Grobbcn (-79) describes for this form an invagination, and also 

 an early differentiation not only of the mesoderm from the 

 endoderm, but also of two distinct portions of the mesoderm. 



