EMBRYOLOGY OF THALASSEMA MELLITA 225 



soon elongate just inside the body wall, and in all probability 

 form later a part of the gut musculature. Another large 

 ectomesoblast cell originates in the b cross region, near the 

 median line (Text-Fig. 9, C, a). This cell arises by delamina- 

 tion, and another from the c cross region, also near the median 

 line, arises by a like process. A small cell sinks in at the same 

 time just in front of the apical plate (Text-Fig. 6, A). These 

 cells also contribute in part to the gut musculature. Conn 

 has found that the various gut muscles of the trochophore per- 

 sist in the adult. He also thinks that some of the mesenchyme- 

 cells become converted into blood-corpuscles. It seems alto- 

 gether probable, too, that the undifferentiated ectomesoblast 

 cells which line the inside body wall of the trochophore (Text- 

 Fig. 8, A) later give rise to the ring musculature. As was indi- 

 cated on p. 208 the floating ectomesoblast cells, without doubt, 

 function as excretory organs. The term "larval," accordingly, 

 applies to very little of the mesenchyme in Thalassenia, for the 

 greater part persists in the adult ; as Meyer ('01) has also found 

 to be case in his studies on the post-larval development of Poly- 

 gordius, Psygmobranchns and Lopadorhynchns. 



It is, indeed, remarkable that the ectomesoblast in Tlialassema 

 from the third quartet agrees so exactly in origin with that of 

 Podarke. The anterior cell in both has exactly the same lineage, 

 and the only difference between the posterior cells in the two 

 forms is that in Tlialassema they bud off an extra small ecto- 

 blast cell before sinking in ; but in Podarke, according to Tread- 

 well, this same division takes place as the cells sink in and the 

 resulting small cells are mesoblast. 



Although this is the first time that a study of the cell lineage 

 of an annelid or mollusc has actually shown a part of the ecto- 

 mesoblast to arise from the first quartet, this possibility was 

 clearly indicated by Kleinenberg's i^^(:>) and Meyer's discovery of 

 neuro-muscular foundations in the praetrochal region of Lopa- 

 dorhynchns. In DinopJiilns, too, the mesenchyme, according to 

 Schimkewitsch ('95), originates by an immigration of ectoderm 

 cells in \.h.Q forzuard part of the embryo. 



Investigations of the cell-lineage of a number of representa- 



