57(5 CKESSWELL SHblAREIJ. 



contains endoderra, remaining' a mesendoblastic cell for eiglit 

 divisions before the niesoblastic is separated from the endo- 

 dermic portion. IMie endodermic part enters into the wall of 

 the arclienterou. In Enpomatus and Podarke the greater 

 part of the mesoblast cell {4d) is mesoblastic, while in Crepi- 

 dnla only a small part of it is mesoblastic. In Amphiti'ite, 

 Mead (26) represents the mesoderm cell similarly giving off 

 a small cell. The spindle of this division, as in Eupomatns, 

 lies in the short diameter of the cell, which at this moment is 

 compressed between the ventral Avail of the ectoderm and the 

 main mass of the invaginated endoderm. The axis of the 

 spindle is in the direction of greatest pressure. 



In Thalassema the mesoblast cells, which at first are 

 pressed together nnder the ventral lip of the blastopore, 

 separate and move apart towards the sides, lying well up 

 towards the prototroch (Text-fig. 9, me.). As they move 

 apart they divide rapidly, each giving rise to a grotip of five 

 or more cells, which form the mesoblast bands as in Annelids. 

 They are quite free in the blastocoel, and enter into close 

 relation with the right and left portions of the larval mesoblast, 

 from Avhich they can be distinguished, as in Enpomatus, 

 by their different staining reaction. Thus Thalassema 

 represents a condition midway between that of Enpomatus 

 and Physa and other Molluscs, where the mesoderm cell 

 lies free in the blastocoel from the time of invagination. 



In Polygordius I have shown (37) that the lioad- 

 kidneys form early and before the mesoderm bands have 

 appeared ; that the rudiments of these organs are first 

 recognisable as two cells in the ventral plate of the ecto- 

 derm. They grow out into the blastocoel, and by division 

 give rise to a string of cells, as in Enpomatus, that run up 

 to the oesophagus. 'J'liey fuse together and become one strand 

 of cytoplasm, with three or more nuclei. This then hollows 

 out, develops a llagellum, and becomes functional as a head- 

 kidney, at an age when the mesoderm bands are represented 

 by a few cells on either side of tlie anal opening-. 



I have advanced reasons for believing that the head-kidney 



