EMBRYOLOGY OF THALASSEMA MELLITA 203 



proctodaeal area at the same time with the closure of the blas- 

 topore, which causes a continuance of the lateral movement 

 and ends finally in the meeting of the edges on the ventral 

 side. In Podarkc, on the other hand, the initiative of the lateral 

 spreading is the fact that the posterior end of the blastopore 

 does not close at all, but is directly transformed into the proc- 

 todaeum. None of these factors are present in Thalass em a dind, 

 as has already been shown, there is nothing to prevent and 

 everything to cause the somatic plate to reach its definite posi- 

 tion very early and by a direct forward shifting. 



The final disposition of the plate is similar to that of Nereis. 

 The side which was at first next to the prototroch in the end 

 reaches a position corresponding to that at which the teloblasts 

 could be last distinguished in Nereis. Wilson ('92) thought it 

 possible that there is a still further shifting until the teloblasts 

 lie at the extreme posterior end of the embryo and form the 

 growing point. It is impossible to determine whether this is 

 the case in Thalassema^ since there are no large teloblasts. 

 This fact, of course, is correlated with the long free-swimming 

 trochophore stage. It is very evident, nevertheless, that before 

 gastrulation the future anal region lies near the prototroch and 

 shifts down during the metamorphosis until it comes in line 

 with the former ^^^ axis, and this involves an evident shifting 

 of the antero-posterior axis (see Text-Fig. 5). This is a simi- 

 lar process to that at work in Nereis^ but is much simpler and 

 possibly more primitive. 



Conn i^'^^i) likewise studied and figured this shifting in Thal- 

 assema, but did not seem clearly to recognize the fact that the 

 final antero-posterior axis coincides with the former ^gg axis, 

 as has recently been found to be the case in a number of anne- 

 lids, and that the shifting of areas is due to the very rapid 

 growth of the dorsal (posterior) side. The distance of the 

 apical plate from the prototroch, on the ventral (anterior) side, 

 on the other hand, undergoes little change. 



It is a difficult matter even to attempt to bring into line the 

 divergent accounts of the axial relations in the development of 

 the trochophore in polychaetes that have been given in late years 



