152 CONKLIN. [Vol. XIII. 



Comparisons. 



In Neritina well-marked bands of mesoblast are formed 

 which have the same axial relations and general appearance as 

 in Crepidula. In the latest stage shown by Blochmann in 

 which these cells appear, Fig. 66, there are four cells in each 

 band and the teloblasts are some distance removed from each 

 other. As has been remarked in another place (p. 74), these 

 cells are connected with each other by several small entoblast 

 cells, Figs. 62, 65, which probably correspond to the entero- 

 blasts of Crepidula, since they are said to form the intestine. 

 As development proceeds the bands move farther down on the 

 sides of the egg and are separated from the small entoblast 

 cells. There is no suggestion in Blochmann' s work of scat- 

 tered mesoblast such as is found in Crepidula. 



The formation and early history of the mesoblast bands in 

 Umbrella were described on page 72. In the later stages 

 Heymons has observed the dissolution of the bands and the 

 nearly uniform scattering of their cells ; at the same time the 

 teloblasts disappear. 



The group of small cells which lies between the teloblasts 

 and corresponds in position, though not in origin, to the entero- 

 blasts of Crepidula " probably gives rise to mesoblast cells 

 which later are found on the outer surface of the intestine." 

 A similar layer of mesoblast cells is found surrounding the 

 intestine in Crepidula, Figs. 80, 81, though I have not supposed 

 that it was derived from the cells lying between the teloblasts, 

 viz., E^, £2, e^, e^. The fact, however, that there is an exactly 

 similar group of cells (the "secondary mesoblast ") in Nereis 

 which has exactly the same fate, and that in Unio also there is 

 a similar group which, as Lillie believes, has the same destiny, 

 is strong corroborative proof of the accuracy of Heymons' 

 observations. In Nereis these cells lie uncovered at the pos- 

 terior lip of the blastopore ; they afterwards become pigmented 

 and migrate inward, where they spread out upon the wall of 

 the mesenteron. Lillie believes that essentially the same thing 

 happens in Unio. In Crepidula this group of cells, four in 

 number, which lies between the teloblasts gives rise, as I 



