No. I.] EMBRYOLOGY OF THE ISOPOD CRUSTACEA. 135 



cells, and may point out that this view is in harmony with those 

 of Samassa ('93) and Nusbaum ('87). Most authors who have 

 studied the development of the Decapods have considered the 

 vitellophags to be endoderm, though Herrick ('92) comes to the 

 conclusion that some of them at least give rise to mesodermal 

 structures. When we come to examine critically the reasons 

 for the belief that they are endodermal, one finds that they do 

 not rest on direct observation, but rather upon analogy with 

 what is supposed to take place in Astac7ts. Here we have an 

 invagination lying behind the region from which the ordinary 

 mesoderm arises, and this is represented in the majority of 

 other Decapods by a plug of cells which give rise to the vitello- 

 phags, and may in certain cases show indications of a cavity, 

 as in Crangon for instance (Weldon, '92). If, then, the invagi- 

 nation of Astaciis gives rise to the mesenteron, as it is sup- 

 posed to do, what more natural than to suppose that the vitello- 

 phags assist in the formation of the endoderm in other forms ? 

 I know of no case, however, in which they have been actually 

 traced to such a final destination, and the whole question comes 

 down to the significance of the invagination of Astacits, grant- 

 ing the homology of the vitellophags with the cells which form 

 the secondary yolk-pyramids of that form. Let us examine, 

 then, the relations in Astaciis, as shown in Reichenbach's 

 beautiful monograph ('86). 



It seems to me that we have to distinguish, according to 

 Reichenbach's descriptions, two sets of elements in the invagi- 

 nation, namely, the cells which absorb the yolk and form the 

 secondary yolk-pyramids, and which, for exact comparison, we 

 may here call the vitellophag cells, and certain cells which do 

 not absorb the yolk, and which form what Reichenbach terms 

 the entoderm plate. From the entoderm plate the liver-lobes 

 arise, and it takes part, also, in the formation of a part of the 

 mesenteron. The important question is how much of the 

 mesenteron is derived from it, and what evidence is there that 

 the yolk-pyramids contribute to the formation of the mesen- 

 teron. In the oldest stage which Reichenbach figures a con- 

 siderable amount of yolk is still present. The stomodaeal and 

 proctodaeal invaginations are well developed, the latter being in 



