1896.] the germinal layers in Am-phioxvs. 153 



Balanoglossus (Q. J. M. S., 1884). The head cavities represent 

 the proboscis cavity, the first myomeres the collar cavities, and 

 the great posterior mesoblastic pouch which becomes segmented 

 into somites the trunk cavity. 



Zoological Laboratory, 

 May 15, 1896. 



(2) Note on the continuity of Mesenchyme cells in Echinid 

 larva?. By E. W. Mac Bride, M.A., Fellow of St John's College. 



About a year and a half ago Mr Sedgwick read a paper before 

 this Society (" On the Inadequacy of the Cell-theory and on the 

 Development of Nerves," A. Sedgwick, Proc. Camb. Phil. Soc, 

 Vol. VIII., Part IV.) in which he strongly insisted on the view that 

 free stellate mesoblast cells such as are commonly figured are 

 not to be found in the Elasmobranch embryo, but that the 

 appearances thus interpreted were really only the thickened nodes 

 of a protoplasmic network. 



Last vacation I had the opportunity of examining numerous 

 larvae of Echinus esculentus, and my observations support those 

 of Mr Sedgwick in a very striking manner. 



The larvae specially referred to were in the gastrula stage and 

 possessed a comparatively narrow alimentary tract, outside which 

 was a wide blastoccele or primary body cavity ; and in this latter 

 were numerous mesenchyme cells. 



These mesenchyme cells were of two main kinds: (1) aggre- 

 gated masses of rounded cells, which formed the matrix of the 

 future larval skeleton, (2) wandering cells or amcebocytes. Of 

 these latter again there were two varieties, of which the first were 

 obviously stellate in form and connected with each other and the 

 walls of the blastocoele by long processes. This kind of cell has 

 been seen and figured by Theel (" The Development of Echino- 

 cyamus pusillus," Hjalmar Theel, Royal Soc. Upsala, 1892), who 

 however regarded the union of the processes of these stellate cells 

 as a secondary phenomenon. 



The second kind of wandering cell is rounded in form, and is 

 in fact precisely similar to the Amcebocyte familiar to all who 

 have studied Echinoclerm anatomy. It has always been described 

 and figured as if it were perfectly free : yet if one examines a 

 slightly compressed living gastrula of Echinus esculentus one 

 observes that these rounded cells, which at first sight look as if 

 they were completely free, are in every case connected either with 

 neighbouring cells or with the walls of the blastoccele by exces- 

 sively fine threads along which they apparently travel. 



