130 MCMURRICH. ■ [Vol. XI. 



So many accounts of the formation of the germ-layers in the 

 Crustacea have been published, and the various accounts have 

 been so frequently brought together in resiim^, that I may be 

 pardoned for refraining from entering into a lengthy, critical 

 review of the Avorks of my predecessors in this line. It may 

 be pointed out, however, that the various accounts may be 

 divided into two groups: (i) Those which derive all the 

 mesodermal and endodermal cells primarily from the blasto- 

 poric cells, and (2) those which assign a portion at least of the 

 mesendodermal formation to extra-blastoporic regions. My re- 

 sults upon the Isopods belong to the first group, and are in 

 harmony, in this respect, with those of the authors, such as 

 Grobben ('79 and '81), Samassa ('93), and Brauer ('92), who have 

 studied the formation of the germ-layers in the lower Crustacea, 

 as well as with those of Bobretzky ('74) on OniscnSy of Bergh ('93) 

 on My sis, of Brooks ('83) on Lucifer, and of Reichenbach (86) 

 on Astaciis, not to mention other students of the Decapods. 

 In the last forms, however, several authors, as for example, 

 Kingsley ('87), Ishikawa ('85), have described the occurrence 

 of cells imbedded in the yolk before the differentiation of the 

 blastoporic cells, a condition probably due to the migration or 

 delamination of some of the blastula cells at an early stage, 

 as has been described by Herrick ('92). This is a phenomenon 

 apparently peculiar to the Decapods, and I do not propose to 

 discuss its significance here; with the exception of this pecu- 

 liarity the mes-endoderm formation of the Decapods is localized 

 in the so-called blastoporic region. 



To the second group belong the observations of several 

 authors whose results require more detailed notice. Nusbaum 

 has described in Mysis iizi) and Ligia ('91) the formation of 

 mesoderm from the ectodermal cells along the entire length of 

 each of the lateral bands of the naupliar region of the embryo, 

 and Lebedinski ('90) holds essentially the same view as to the 

 origin of the mesoderm in EripJiya. As regards Mysis Bergh 

 has given a very positive statement as to the incorrectness of 

 Nusbaum's observations on this point, and as already stated 

 there seems to me to be no evidence in the Isopod that such a 

 mode of formation of the mesoderm obtains. Nusbaum evi- 



