150 CONK LIN. [Vol. X 1 1 1 . 



about the stage shown in Fig. 52, one additional mesoblast 

 cell in each of the quadrants A, B, and C, lying immediately 

 below the ectoblast cells at the angles of the quadrangular 

 blastopore. As was shown on page 129, these ectoblast cells are 

 derivatives of the second quartette; and since the additional 

 mesoblast cells are derived from the overlying ectomeres, it 

 follows that they have come from the cell groups 2a, 2b, and 

 2c, though I cannot give their exact cell origin. In the few 

 cases which I have been able to examine, the additional meso- 

 blast cell is formed first in quadrant A, and afterward one is 

 formed in each of the quadrants C and B. In C and B they 

 lie on the right (as seen from the vegetal pole) of the cells 4c 

 and 4b, while in quadrant A the mesomere lies to the left of 

 the cell 4a. In other words, these additional mesoblast cells 

 are bilaterally placed in the quadrants A and C. There is no 

 doubt that the scattered mesoblast above described comes 

 from these additional cells. 



These additional mesoblast cells forcibly recall the " larval 

 mesoblast " of Unio. Like the latter, they are formed from 

 the second quartette of ectomeres, they are not teloblastic in 

 growth, but give rise to scattered mesoblast cells, and they seem 

 to be concerned chiefly in the formation of unicellular muscle 

 fibres, or myocytes,^ which appear in the foot and in those lar- 

 val organs, the head vesicle and velum. On the other hand, 

 these cells differ from the larval mesoblast described by Lillie 

 in the fact that they arise in three quadrants instead of in one 

 only as in Unio, they appear at a much later stage, and they 

 probably give rise to adult as well as to larval structures. 

 These differences, however, are of secondary importance as com- 

 pared with the resemblances mentioned, and I do not doubt 

 that these cells correspond to the larval mesoblast of Unio. 



The origin of larval mesoblast in three quadrants is most 

 suggestive, since it points, as I believe, to a primitively radial 

 origin of the mesoblast. From every point of view it seems 

 probable that Crepidula represents a more primitive condition 

 in this regard than Unio. The radial symmetry of the other 

 layers is more complete in Crepidula than in Unio and is pre- 



1 1 follow Lillie ('95), p. 38, in the use of this term. 



