158 CONKLIN. [Vol. XIII. 



In Fig. 89, which is a median horizontal section through an 

 embryo of the stage shown in surface view in Fig. 74, these 

 cells form the posterior boundary of the mesenteron. In the 

 vertical, longitudinal section shown in Fig. 92 it is also seen 

 that they form the ventral wall of the intestine. The position 

 and histological character of these cells leaves little room for 

 doubt that they have been derived from the enteroblasts. 



After the yolk cells have moved over to the ventral side of 

 the mesenteron there is left a polygonal opening between 

 them at the posterior end, and by means of this opening the 

 intestine communicates with the cavity inclosed by the large 

 yolk cells. Fig. ']6. By the laeotropic torsion to which the 

 posterior end of the embryo is subjected, this opening is car- 

 ried farther and farther up on the right side of the embryo, 

 Figs. 6Z, ^6, 80, 81, and at the same time the intestine grows 

 longer, partly by the division of the enteroblasts and partly by 

 the addition of cells which are derived from the yolk cells. 



At this stage none of these small intestinal cells contain 

 yolk spherules, and yet they do not stain like the purely proto- 

 plasmic cells of the ectoderm or mesoderm, being lighter in 

 color and more transparent. At an earlier stage the entero- 

 blasts contained yolk spherules. Figs. 22-52, but in these 

 later stages they seem to have been dissolved. 



The ventral wall of the intestine is from the first composed 

 of these small cells. Figs. 76, 92, while the dorsal wall is 

 formed by the large yolk cells ; the intestinal cells, therefore, 

 form a groove, open widely on its dorsal side to the yolk. The 

 two sides of this groove arch over and meet each other first at 

 the distal end of the intestine and then progressively forward 

 from this point ; the intestine thus becomes a tube with a dis- 

 tinct lumen, the walls of which are composed entirely of small 

 intestinal cells, Figs. 95, 104, 105. At its anterior end, how- 

 ever, where it opens into the cavity between the yolk cells, the 

 intestine remains a groove. Figs. 93-103. 



The posterior opening between the yolk cells, at which point 

 the intestinal cells arise, lies at first on the ventral mid line. 

 In the course of farther development, however, the yolk cells 

 shift their positions, carrying this point of origin of the intes- 



