462 HAROLD HEATH 



to the ectodermal elements generally. Furthermore all of the 

 cells of the apical plate are packed with granules, evidently 

 yolk, and thus are in marked contrast to the remaining ectoderm 

 where the granules are relatively few and minute. 



Generally speaking, this condition of affairs continues to the 

 close of gastrulation. In specimens entering upon the initial 

 stages of enterocele formation, where the walls of the blind end 

 of the archenteron have grown thin, a few of the cells of the 

 apical plate are usually to be seen in the act of migrating into 

 the blastocele. As will be seen later these and other migrants 

 from the apical plate form mesenchyme. 



The formation of the enteroceles (hydroceles) proceeds in the 

 typical fashion. In practically every case they arise independ- 

 ently of each other, and from the outset the left is almost in- 

 variably the larger of the two (fig. 1). Mesenchyme cells arise 

 as usual from the walls of the enteroceles as well as from the 

 intervening portion of the archenteric wall. Their numbers, 

 when compared with those in Asterias ocracea for example, are 

 relatively small, and, in comparison with the migrating cells of 

 the apical plate, they are distinctly smaller and far more hyaline 

 in appearance — features which enable one to distinguish the 

 two types of mesenchyme (ectodermal and entodermal) through- 

 out the stages described in this paper. 



In fully 50 per cent of the specimens in hand a few of the mesen- 

 chyme cells arising from the blind end of the archenteron, be- 

 tween the enterocele pouches, unite to form a small vesicle (fig. 

 1, av). This usually occurs after the enterocele pouches are well 

 differentiated though not completely cut off. In a relatively 

 short space of time this anterior, unpaired vesicle is rapidly 

 enlarged by the addition of other mesenchyme cells from the 

 archenteron, and ultimately attains a diameter equal to that 

 of the gut. In a few surface views it is clearly seen to be entirely 

 enclosed, with walls everywhere complete, and, though it is 

 closely attached to the distal wall of the archenteron, it is never- 

 theless distinctly separated from it. In other cas6s the vesicle 

 is so closely applied to the archenteric wall that it is not pos- 

 sible to determine whether the vesicular walls are complete. In 



