76 CONKLIN. [Vol. XIII. 



respond to 4a and 4c are formed almost immediately after 4d, 

 and before the latter divides into right and left halves. At a 

 later stage two other cells, designated by Blochmann en^ and 

 €71^, are supposed to have come from the macromere a (B of 

 our system). In Blochmann's figures they are shown in the 

 segmentation cavity, half way between the animal and vegetal 

 poles. All of these cells, together with en^, a small entoblast 

 cell whose origin was not known, are shown in the figures ^ 

 moving up through the space between the macromeres into the 

 segmentation cavity on the upper side of the &gg. It seems a 

 very remarkable thing that entoblast cells should travel through 

 the segmentation cavity in this way. So far as I know, nothing 

 like it occurs in any other animal, and I find it hard to believe 

 that Blochmann is right on this point. There are too many 

 points of agreement between Crepidula and Neritina through- 

 out the entire development to make probable the view that they 

 are so wholly unlike in this one regard. Only one figure of the 

 small entoblast cells given by Blochmann has a familiar appear- 

 ance, and that is his Fig. 64, in which three small entoblast 

 cells are shown at the vegetal pole in the positions occupied by 

 4a, 4b, and 4c in Crepidula. 



These fourth-quartette entomeres were observed and figured 

 by McMurrich ('86) for Fulgur, though he did not suspect their 

 real nature. He says of them (p. 413): "On surface view 

 three elongated elevations (Plate XXIV, Fig. 8) are seen radi- 

 ating toward the centre of the blastodermic area, but not 

 extending centrally farther than the edge of the area, and lying 

 rather alternate with the macromeres than opposite them. 

 What the significance of these elevations may be it is not easy 

 to say, but sections through ova of this stage show them to be 

 coincident with the first formation of the mesodenn. ... If 

 this interpretation of the sections be correct, it would seem that 

 the macromere which does not show an elevation on surface 

 view is the one which gives rise to the mesoderm, but what may 

 be the cause of the formation of the elevations on the macro- 



1 In interpreting Blochmann's account of these smaller entoblast cells I have 

 been compelled to rely largely upon his figures, since little mention is made of 

 them in the text. 



