1918 Leonard: on Haminea 51 



spect." If this comparison is applied to Haminea, in which the first 

 quartet divides once simultaneously with the formation of the second and 

 again between the stages of 2i and 29 cells, as it does in Crepidula, we 

 may expect it also to be intermediate in the rate of development of the 

 upper hemisphere. 



The second quartet of micromeres come to rest in the furrows between 

 the macromeres producing them, each in the furrow to the right of the 

 macromere producing it, thus pushing the cells of the first quartet back 

 to a position directly above the macromeres which produced them. The 

 turret cells lie in the furrows between the cells of the second quartet and 

 thus almost directly over the macromeres (Figs. 16, 17). Conklin says: 

 "In this case as in every other which I have observed, the spiral character 

 of the cleavage is much more pronounced after the nuclear division than 

 during that division. It seems to be a phenomenon belonging to and 

 caused by the cytoplasm rather than the nucleus." 



FIFTH CLEAVAGE 



The two divisions which constitute the fifth cleavage divide the macro- 

 meres, A-D, and the second quartet of micromeres, 2a-2d, dexiotropically 

 {Figs. 17, 18). The division of both quartets is essentially equal, the 

 micromere quartet giving rise to a lower moiety, 2a"-2d", and an upper 

 moiety, 2a^-2d^ ; the macromeres giving off the third quartet of micromeres, 

 3a-3d. Division in the micromere quartet tends to precede slightly that in 

 the macromeres {Fig. 17). 



At the close of this cleavage the egg consist of 24 cells: 8 cells 

 Oa^-ld^ and la--2d") products of the first quartet; 8 cells (2a^-2d^ and 

 2a--2d") products of the second quartet; the 4 cells of the third quartet; 

 and 4 macromeres. The cells of the tliird quartet, which are about equal 

 in size to those of the second, lie in the furrows between the macromeres, 

 each to the right of the macromere producing it. The lower moiety of the 

 second quartet lies just to the lift of the mid-line of the macromere be- 

 neath, the upper moiety lies just to the right. At this stage, as in the 

 earlier stages, the cells of the Haminea egg do not flatten against each 

 other as do those of Planorhis (Holmes 1900) or of Crepidula (Conklin 

 1897), although this phenomenon is less marked in Crepidula than in 

 Planorhis. A side view of the Haminea egg at this stage shows each of 

 the clear protoplasmic cells fully rounded on the free surfaces and flat- 

 tened only where in actual contact with an adjoining cell. 



SIXTH CLEAVAGE 



Following the division resulting in the 2 1-celled egg the cells of the 

 first quartet again divide dexiotropically, giving rise to l small cells below, 



