130 CONKLIN. [Vol. XIII. 



From this time on the blastopore closes from the sides more 

 rapidly than from the anterior and posterior ends, and as 

 a consequence the quadrangular shape is lost, and the blasto- 

 pore becomes an irregular oval, Figs. 54, 57, 61, and then an 

 elongated slit-like opening. Figs. 58, 60, 63. Finally both the 

 anterior and posterior ends of the slit close, and there is left 

 a narrow pore, Figs. 65, 66, yi, 72. Immediately around this 

 pore there is a depression of the ectoblast, Figs. 65, 73, which 

 is most extensive on the anterior and lateral sides The out- 

 lines of this depression become sharply marked, forming the 

 fundament of the mouth; and its inner edges, especially the 

 two lateral boundaries, turn inward as shown in Fig. 6?,, form- 

 ing the fundament of the oesophagus or stomodaeum. For a 

 brief period the stomodaeum is closed at its inner end, Fig. 88; 

 but it soon opens again at the very point at which it closed, 

 Figs. 90 et seq., and thereafter remains in open communication 

 with the cavity of the mesenteron. It is at first very short. 

 Figs. 90, 91, but later becomes a long tube. Figs. 92, 93, 95. 

 When it first begins to elongate, it is directed anteriorly from 

 the mouth-opening to the mesenteron, so that it opens into the 

 anterior part of that cavity. Fig. 92. In later stages, with the 

 growth of the foot, the expansion of the shell gland, and 

 the enlargement of the whole region posterior to the velum, the 

 mouth-opening is pushed farther and farther forward, the yolk 

 cells are shifted backward, and the whole direction of the stomo- 

 daeum is reversed, so that it runs posteriorly from the mouth- 

 opening to the mesenteron. Figs. "/?:, 93, 95. Finally, with 

 the greater development of asymmetry the inner end of the 

 stomodaeum is moved slightly to the right, as shown in Fig. 82. 



Throughout its entire length the stomodaeum is composed of 

 columnar, ciliated cells, and along its posterior wall there is a 

 double row of large clear cells, with cilia larger than usual, 

 which is directly continuous with some large ciliated cells cover- 

 ing the median surface of the foot ; by the beating of these 

 cilia the nutrient fluid surrounding the embryo is drawn into 

 the mesenteron, Figs. 99, 104, 105. 



(b) The Posterior Grozving-Point. — At first the cell divi- 

 sions on the posterior side of the tg^ are less frequent than on 



