No. I.] THE EMBRYOLOGY OF CREPIDULA. "jCj 



mesoblast, and • it is not until the three smaller entoblasts are 

 formed and the whole of the ectoblast has been rotated in an 

 anti-clockwise direction that the planes of symmetry in the 

 three layers come to coincide in the median plane of the future 

 animal.^ The bilateral symmetry which is to characterize the 

 adult appears at different times and in different directions in 

 each of the layers, and at a later period these planes, which 

 have been diversely established, come to coincide in the chief 

 axis of the developing organism. No better evidence could be 

 desired to show that such forms of cleavage are coenogenetic, 

 and that at the same time they are not the result of merely 

 mechanical causes. In cleavage, as in the entire ontogeny, one is 

 impressed with the evident purposefulness of every eve7it ; the end 

 seejns to be in viezvfrom the beginning, and the building ■materials 

 are sorted and arranged zvitJi reference to this end result. 



In this connection it is interesting to inquire into the causes 

 which produced this rotation. There can be little doubt that it 

 is due to the three smaller entoblasts, since at this time there 

 is no apparent activity in any other part of the ovum. These 

 cells are given off in a left spiral cleavage, and they lift the over- 

 lying ectoblast cells and turn them in an anti-clockwise direction 

 until these three entoblasts lie in the furrows between the macro- 

 meres, and so the egg is left in as compact a form as possible. 



Heymons ('93) has shown that an exactly similar rotation of 

 the ectoblast takes place in Umbrella. The rotation occurs at 

 the same time, in the same direction, and to the same extent 

 as in Crepidula. Heymons also assigns the same cause which 

 I have attributed both here and in my former paper ('92), viz., 

 the rotation of the small entoblasts into the furrows between 

 the macromeres. 



Fig. 33 seems to indicate that the primary enteroblasts, E' 

 and E^ were prevented from rotating into the furrow between 

 D and C by the pressure of the overlying cells, for as soon as 

 the latter are lifted by the formation of the smaller entoblasts 



1 It is not strictly true that the planes of symmetry in the three layers coincide 

 after the rotation of the fourth quartette, though they are brought much nearer 

 together by that rotation. Even in the stages immediately preceding the formation 

 of the larva, Figs. 65-76, it can be seen that the apical cells of the ectoderm, Ap., 

 still lie to the right of the median plane in the entoderm. 



