202 TORREY 



trochal region seems to be much greater than it is in TJialas- 

 sema and the "/" cells become very much larger in the for- 

 mer than in the latter. In Thalassenia any extensive lateral 

 spreading of these cells is prevented by the fact that the X 

 group reaches the prototroch on each side (PL II, Fig. 24). 



Shifting of Areas (Text-Fig. 5). — The divisions of the X 

 group to the beginning of gastrulation have already been de- 

 scribed. The spreading of the group now continues rapidly, 

 not only by cell division, but also by a swelling of the cells 

 near the prototroch. These processes are all clearly morpho- 

 genic. Soon after the closure of the posterior end of the blas- 

 topore, the cells anterior to it become shoved around the open 

 anterior end by the movement of the ventral plate toward 

 the future ventral side (PI. II, Fig. 24; Text-Fig. 4, D). 

 At length some of the larger cells on the mid-ventral line sink 

 into the opening and latei take part in the formation of the 

 posterior wall of the oesophagus (Text-Fig. 6, B, C and D). 

 Owing to this forward migration, and in consequence ot the 

 great flattening and diminution in area of the post-trochal re- 

 gion, the ventral plate now extends to the stomodaeal opening 

 (Text-Fig. 5, D). A still further shifting of this plate is brought 

 about both by the growth of the cell group which has migrated 

 from the prae -trochal region, and by the elongation of the body 

 along the future antero-posterior axis (Text-Fig. 5, ^ and F)- 



The X group gives rise to by far the greater part of the post- 

 trochal ectoderm of the trochopore. A small ventrolateral re- 

 gion (Text-Fig. 5, i^) is formed by the cells which do not enter 

 into the differentiation of the oesophagus. They are certainly 

 2^:^ 2. 2 ^^d 2<ri 2. 2 ^^''d possibly also '^d^^ and 3r^.i. 



The behavior of the ventral plate in Thalasseina is different from 

 anything that has hitherto been described in annelids. There 

 is no concrescence of the lateral edges on the ventral side, as is 

 the case in Nereis, Ainphitrite, Arenicola and Podarke. In the 

 first three cases two factors may be said to cause this phenom- 

 enon. The first is the rapid development of the somatic plate 

 before the closure of the blastopore, which necessitates its spread- 

 ing laterally only. The second is the formation of a terminal 



