198 TORREY 



this movement lies in the closest association with the formation 

 of the oesophagus. The cells, accordingly, which finally line 

 the blastopore are the following : at the posterior end .r^ 0+ on 

 the right side the progeny of 3^2.0, 3<^2. 1. 1> 3<^2. 1.2. 2) on the left 

 side that of 3<:/2. i> 3'^2. 2. 2. 2. 3^2. 2.1 5 ^^^ at the rounded ante- 

 rior end that of 2^2.;2. 2» 3^2. 2. 2» 2^2.2.2 ^^^ the large oesophago- 

 blast 2^2. 2. 1. 2- ^^ Areiiicola the cells of the second quartet are 

 shoved away in a similar manner from the sides of the blasto- 

 pore by cells from the c and d quadrants of the third quartet. 



The blastopore, when fully formed in Thalassemia, is an elon- 

 gated slit with an enlargement at the anterior end (Text-Fig. 4, 

 C). This latter part 7iever closes, but after the formation of the 

 cesophagus becomes the mouth. The subsequent movement of 

 the blastopore toward the future ventral side is brought about, 

 on the one hand, by the forward movement of the ventral plate 

 {X group), which in turn is caused by the migration of cells 

 from the prae-trochal region through the dorsal gap. On the 

 other hand an equally important factor in the shifting of the 

 blastopore is the simultaneous insinking under the prototroch 

 of the large anterior oesophagoblast (2/722.1.2. PI- II> Fig- 24). 

 The posterior part of the blastopore finally becomes closed by 

 the approximation of the sides. The closure seems to be 

 brought about, not so much by active cell divisions as by the 

 fact that the post-trochal region becomes greatly flattened as a 

 consequence of the insinking of the entoblasts, the 7^/ cells, and 

 the ectomesoblasts from the third quartet. This flattening 

 would naturally tend to bring the edges of the elongated blasto- 

 pore together. The open anterior end at length comes to lie 

 just under the prototroch (Text-Fig. 4, D) — the position of the 

 future mouth. The closure of the blastopore from the sides 

 reminds one in some degree of the process in Luinbriciis (Wilson, 

 '89). In cases where the macromeres are very large and fill 

 the cleavage cavity, as in Nereis and AinpJiitrite, the closure 

 takes place at the lower pole by the convergence of the cells 

 from all sides. In Capitella (Eisig, '98) the blastopore is said 

 to close completely, and this is probably the case in the ma- 

 jority of annelids. 



