1918 Leonard: on Ilaminea ID 



guishable. Of the four blastomeres (A, B, C. U) formed by this cleavage. 

 B and D meet in the cross-furrow at both poles; the other two (A and C) 

 being entirely separated. 



The 3-celled stage produced by the difference in time of cleavage of 

 AB and CD, which is typical of Umbrella (Heymons 1893), and of Tethys 

 (Viguier 1898), does not appear in Haminea. Heymons finds in Umbrella 

 and Viguier in Tethys a typical cross-formation in the 4-celled stage, in 

 that the two cells homologous with A and C {Fig. 9) characteristically lie 

 above the other two cells B and D. Warneck (1850) reports the same 

 phenomenon for Limnea and Lang (1884) for Discocoelis iigrina, in 

 which the two smaller blastomeres lie above the two larger. This situa- 

 tion does not occur in Haminea, although it would at first appear so in 

 viewing the egg from the animal pole, on account of the diagonal direction 

 of the cleavage planes which cut the blastomeres essentially into an upper 

 and a lower moiety, that is, so that the cells A and C overlap the cells 

 B and D. That they do not actually lie above them as in Umbrella and 

 Tethys is demonstrated in a side view of this stage {Fig 10), in which it 

 may be clearly seen that all four cells touch the same plane at both upper 

 and lower poles. 



THIRD CLEAVAGE 



The third cleavage is the first segregating cleavage, separating four 

 )3rotoplasmic micromeres from the four yolk-laden macromeres. The 

 spindles lie with the inner ends higher than the outer, and vary from a 

 nearly radial {Fig. 11) to a markedly spiral {Fig. 12) position. Tliis is 

 in close agreement with Crepidula (Conklin 1897) in which the spindles 

 are "usually nearly radial in position, though frequently slightly inclined 

 in a right spiral and occasionally even in a left spiral." Whether, as in 

 Crepidula, there is a dexiotropic rotation of the upper poles of the spindles 

 during division has not been determined. The most markedly arranged 

 spindles were found in the earliest stages of division {Fig. 12). The 

 posterior cells, C and D, tend to precede A and B slightly in the forma- 

 tion of the spindles {Fig. 12), altho cell division appears to be practically 

 .synchronous. 



The cleavage is dexiotropic and unequal {Fig. 13). The four micro- 

 meres {la, lb, Ic, Id) formed by it come to rest in the furrows between 

 the macromeres (A, B, C, D). their contours becoming modified to fit 

 more closely between the cells {Fig. IJf). Wlu'tlur the dexiotropic rota- 

 tion of the micromeres or any part of it is accomplished after division is 

 complete the writer has been unable to determine on the material at hand ; 

 but there is evidence of the rotation continuing after complete separation, 

 since in no instance has a spindle been found having a dexiotropic inclina- 



