No. I.] THE EMBRYOLOGY OF CREPIDULA. 159 



tinal cells anteriorly and on to the right side of the embryo, 

 Figs. ^6, 80, 81. The blind distal end of the intestine 

 remains for a long time on the ventral mid line, while the 

 main portion of the intestine curves up over the body of the 

 embryo anteriorly and to the right, running just beneath 

 the ectoderm until it ends in the shallow groove of intestinal 

 cells which opens into the cavity between the yolk cells. 



A series of horizontal, longitudinal sections through an 

 embryo of the stage shown in Fig. 80 is given in Figs. 102- 

 105. Fig. 102 is taken through the plate of intestinal cells 

 at the upper margin of the yolk-opening. Fig. 80; Fig. 103 

 is taken through the middle of that opening; while Figs. 104 

 and 105 are below that opening and show the intestine as a 

 tube ; at a little lower level it ends blindly. A thin layer of 

 mesoderm cells surrounds the intestine in the later stages, but 

 is usually separated some distance from its walls, Figs. 80, 

 81. 



In older larvae the torsion goes still farther, the central end 

 of the intestine being carried dorsally and posteriorly, and the 

 distal end being brought up onto the right side of the embryo, 

 where it ultimately opens into the branchial chamber. The 

 anus does not appear until very late, — later than any of the 

 stages figured, — and the proctodaeum is extremely short, 



(c) The Stomach. — That part of the mesenteron which is 

 bounded by yolk cells we may perhaps, after the example of 

 Bobretzky ('77), call the " stomach," though it gives rise to 

 other structures besides that organ. The yolk cells which 

 bound its cavity are at first large and pyramidal, their bases 

 being outward and their apices extending into the cavity of 

 the mesenteron. The protoplasm of each cell is aggregated 

 at the apex, where also the nucleus is located, while the rest 

 of the cell is densely packed with yolk spheres. At first the 

 ectoderm cells at the inner end of the stomodaeum abut 

 directly against some large yolk cells. Fig. 92, probably the 

 derivatives of 4A, 4B, and 4C ; but as development proceeds, 

 the entoderm cells in this region divide repeatedly, growing 

 much smaller, though they are still filled with yolk, while 

 farther back the cells forming the walls of the stomach cavity 



