126 MCMURRICH. [Vol. XI. 



Such being the mode of formation of the digestive tract, 

 what is the ultimate fate of the vitellophags, and which of the 

 three germinal layers do they represent } 



At the time of the migration of the blastopore cells the 

 vitellophags sink into the yolk, which divides up into masses 

 as has been so frequently described in other Crustacea, there 

 being, however, no formation of secondary yolk pyramids such 

 as occur in Astacus. The yolk persists for a long time, even 

 up to hatching in the thoracic region of the body ; but it dis- 

 appears in slightly earlier stages in the abdominal region and 

 here phenomena which accompany its disappearance may be 

 studied. In Fig. G'j is represented a transverse section through 

 the anterior portion of the abdomen of an embryo of Cymot/ioa 

 in which the yolk is beginning to undergo disintegration. 

 Towards the ventral surface on either side is to be seen one of 

 the lateral mesoderm masses whose cells are beginning to be 

 converted into muscle tissue {mu), and in their vicinity are to 

 be seen a number of scattered cells, whose origin is I believe 

 indicated on the left side of the figure. Here at the sides 

 above the lateral mesoderm mass are a number of vitellophags 

 still scattered through the yolk, which throughout the whole 

 section has broken up into small and somewhat separated 

 particles, a preliminary to its breaking down into minute granu- 

 lar particles such as are seen in the upper left-hand portion of 

 the figure {dy). As this ultimate disintegration occurs the 

 vitellophags are set free and form the scattered cells already 

 alluded to. This setting free of the vitellophags is not confined 

 to the lateral regions of the body, but is to be seen also towards 

 the dorsal surface. Here one finds in the section figured in 

 the middle line the proctodaeum {pr) and on either side of it. is 

 a row of cells which correspond to Nusbaum's cardioblasts. 

 Their situation in the yolk seems to indicate that they may 

 represent vitellophags, though I have not been able to trace 

 either their origin or their ultimate fate. Above the procto- 

 daeum and on either side are to be found numerous other cells, 

 some of which and probably all are freed vitellophags, and 

 various stages in their aggregation and separation from the 

 yolk may be found by tracing the series of sections forwards. 



