48 CONKLIN — EMBRYOLOGY OF A BRACHIOPOD. [April 4. 



is always but one cell thick (Figs. 40-42), though in the earlier 

 stages of its development it represents a fold in the wall of the 

 archenteron (Figs. 42^^-42^). The backward growth of this par- 

 tition wall continues until the enteron is entirely separated from 

 the enterocoel save for a narrow slit-like communication at the 

 posterior end (Figs. 20, 24). 



While the enteron is thus being separated from the enterocoel 

 the blastopore is gradually closing and the whole embryo is becom- 

 ing flattened in a dorso-ventral direction and elongated antero- 

 posteriorly. The blastopore is at first a circular opening ; it then 

 becomes narrowed from side to side and apparently elongated 

 antero-posteriorly (Figs. 17-21). The blastopore groove thus 

 formed is shallow posteriorly and deepest at its anterior end where 

 it opens into the enterocoel and enteron (Fig. 17). This groove 

 continues to grow narrower and to be filled up at its posterior end 

 until it becomes a mere slit, opening by a small pore near its 

 anterior end into the enterocoel (Figs. 21, 22). Finally this pore 

 also closes (Fig. 24) and the enterocoel and enteron are completely 

 shut off from the exterior, though still communicating with each 

 other by a narrow opening in the region posterior to where the 

 blastopore closed (Figs. 24 and 46, 47). The blastopore groove 

 persists for some time after the pore has closed but ultimately dis- 

 appears, though a depression is left at the anterior end of this 

 groove which becomes a part of the anterior mantle furrow ; it is 

 probable that at this very point the oesophageal invagination occurs 

 at a stage after the fixation of the larva (see p. 56). 



In stages in which the blastopore is still circular the enterocoel 

 is but little larger than the enteron (Figs. 14 and 38). In looking 

 at an entire egg of this stage from the oral side one sees two cavi- 

 ties of about the same diameter, one above the other, which com- 

 municate with each other by a wide opening ; the cavity nearest 

 the blastopore is the enterocoel, the one nearest the aboral side the 

 enteron. In an older stage (Fig. 17) in which the blastopore has 

 begun to narrow one still sees that these two cavities are of nearly 

 the same diameter. As the enteron becomes separated from the 

 enterocoel, however, the latter becomes much more extensive than 

 the former, and an oral view of an embryo at this stage shows the 

 enterocoel lying on the oral side of the enteron and entirely 

 surrounding it except on the aboral side (Fig. 21, also optical sec- 

 tions, Figs. 19 and 20). This rapid enlargement of the enterocoel 



