I 2 4 MCMURRICH. [Vol. X I . 



embryos of all the Isopods I have studied, but I must take 

 exception to Nusbaum's interpretation of them. In prepara- 

 tions of entire embryos, such as he figures, they might readily 

 be taken for ectodermal thickenings, but sections show at once 

 that they are really the lateral mesodermal masses shown in 

 my Figs. 65 and 66 {IM). An homology of these structures 

 with epipodites seems out of the question ; they do not stand 

 in relation to the limb, but are situated in the pleura when 

 these are developed, and, as stated, give rise to the lateral 

 muscles of the body. 



So far nothing has been said regarding the origin of the 

 heart, practically all the teloblastic mesoderm going to form 

 muscle-tissue. Inasmuch as the heart formation concerns the 

 vitellophag cells, a description of its origin will be postponed 

 to the next part of the paper, in which the fate and significance 

 of the vitellophag cells will be discussed. 



2. The Formation of the Digestive Tract and the Later History 



of the Vite Hop hags. 



It has been stated in an earlier part of the paper that what 

 are probably to be considered endoderm cells are distinguish- 

 able in an early stage oi faera and perhaps also of Asellns, but 

 they are recognizable only by their position and not by any 

 histological peculiarities. In the Oniscidas studied they could 

 not be determined, the entire aggregate of mesodermal cells 

 being identical in appearance. Even in the Asellids they 

 become indistinguishable in the later stages when the scatter- 

 ing of the mes-endoderm takes place, and it has not been 

 possible to trace their later history and their conversion into 

 the liver-lobes, which is, I believe, their fate. 



However that may be, the liver lobes first make their appear- 

 ance as a mass of cells on either side at about the level of the 

 first maxillary segment, and the cells of each lobe early arrange 

 themselves in an epithelium, forming a more or less spherical 

 body which is open towards the yolk, a certain amount of 

 which is enclosed by them and apparently undergoes within 

 them a digestion. At an early stage in Cymothoa the spherical 



