622 



HAROLD HEATH, 



archenteron, but there is the strongest reason to believe that they 

 form part of the stomodaeum and are consequently ectodermal cells. 

 As these latter press in toward the vegetative pole the macromeres 

 pass completely within the embryo. The third quartette now divides 

 a second time, forming two cells that are the homologues of Sa^- ^, 

 3a^-'^ etc. in Ischnochiton (diagram D). 



Fig. D. Two stages in tlie gastrulation of Chiton marmoratus and C. squamosus, as 

 figured by Metcalf. The nomenclature is the one used in this paper. 



In each quadrant between the stomatoblasts is a small cell 

 marked "?" whose origin was not accurately determined by Metcalf 

 though it was thought that they correspond to the fourth quartette. 

 This is probably correct and the cells therefore correspond to 4 a, 

 41) etc. (they are so marked in diagram D). If the view be sound 

 that these cells are the homologues of the fourth quartette then one 

 of them must be the mesoblast. That this interpretation is correct 

 is shown, I believe, by one of Metcalf's figures represented in Db. 

 Invagination here has progressed further than in Fig. a of the same 

 diagram yet the cells maintain essentially the same positions. One 

 of the cells, a fourth quartette product has divided. Comparing this 

 with Fig. 35 practically the same conditions prevail. The only dif- 

 ference is that the third quartette stomatoblasts, 5 a -2 etc., have not 

 divided. In Ischnochiton also one of the fourth quartette cells (meso- 

 blast) has formed two cells. The resemblance seems too striking to 

 be a mere coincidence and the cells in their origin and history up to 

 this time are the same, neglecting minor differences, and I believe 

 therefore that in diagram D Metcalf has figured the mesoblast. 



Metcalf also mentions a furrow in the mid-ventral line extending 

 anteriorly from the blastopore out toward the prototroch and 



