The Development of Theridium. 307 



posterior cumulus the group of them that formed there. This move- 

 ment of vitellocytes from their point of origin comes to lower the 

 elevation of the anterior cumulus and to cause it to become flush with 

 the surface of the germ disc. 



At the same time mesentoblast cells are scattered beneath the germ 

 disc except at its posterior pole (for they do not appear to arise there). 

 These are polygonal and relatively small, placed between the outer 

 cell layer and the yolk or yolk cells ; they are lettered Mes. E. in Figs. 

 41bc, PI. Ill, and 42b, 43a and 43b, PL IV. They do not compose 

 a continuous layer and are only one cell deep except at one point, 

 where they are two deep (Fig. 41c, PL III). I have searched care- 

 fully but in vain to find indications that these mesentoblast cells de- 

 velop in situ from the overlying ectoblast ; all the mitotic spindles of 

 the ectoblast seem to be placed horizontally and none vertically at 

 this stage. Therefore, it is probable that the mesentoblastic elements 

 of this stage are emigrated descendants of that group of six to eight 

 cells of the previous stage {Mes. E., Fig. 37a, PL III) which formed 

 part of the anterior cumulus. They seem to have wandered from a 

 single point of origin, just as the vitellocytes have done. Definitive 

 entoblast and mesoblast will later form from this mesentoblast, as 

 will be described in due time. There is closure of the gastrocoel at 

 this period, one of several indications that gastrulation is ending and 

 consequently cellular invagination, and there is no indication at any 

 later stage that either mesoblast or entoblast forms from the outer cell 

 layer; therefore, the latter from now on may be termed ectoblast. 

 The ectoblast cells are becoming higher than wide. Thus, the germ 

 disc is at many points two-layered, consisting of outer ectoblast and 

 inner mesentoblast, both placed outside of the vitellocytes. 



Still another process is under way, namely, formation of vitello- 

 cytes from the anterior and lateral margins of the germ disc, an 

 origin quite independent of the centers of formation represented by 

 the two cumuli. Their formation from the anterior margin of the 

 germ disc is shown in Figs. 30a, b, 38a, 41a, PL III, and from the 

 lateral margin in Figs. 31 (an unusually pronounced case) and 39. 

 Whether this is effected by vertical mitoses or by inrolling of the edge 

 of the germ disc I have not positively determined, but there are indi- 



