1918 Leonard: on H amine a 17 



tion of the first cleavage spindle the zonal arrangement has become decid- 

 edly marked. 



FIRST CLEAVAGE 



It was not observed how long after fertilization or extrusion the first 

 cleavage occurs. The first cleavage spindle forms at right angles to the 

 plane of the polar spindle, at a point a little more than I/4 the diameter 

 from the animal pole (Fig. 4)- The zonal distribution of the yolk mate- 

 rial is marked even in the unstained specimen, the vegetal hemisphere 

 being dense and opaque with the aggregation of deutoplasm spheres, the 

 upper hemisphere clear except for a fine granular network which runs 

 throughout the egg. 



The cleavage of the cytoplasm begins as a broad depression at the 

 animal pole, gradually extending around the entire egg and becoming 

 deeper and narrower until separation is complete (Fig. 5). As cytoplasmic 

 division begins, the spindle apparently rotates slightly so that the end 

 which is to enter the smaller of the two daughter cells lies higher than the 

 other, thus indicating a spiral trend of cleavage. Since my work has all 

 been done on preserved material I have not been able to observe the process 

 of this rotation, but preparations of the earlier and later stages of the 

 first cleavage indicate clearly that such a rotation does take place after 

 the formation of the first nuclear spindle (Figs. 4> 5)- The cleavage is 

 unequal, dividing the egg, in the nomenclature of Conklin (1897), into the 

 two cells AB and CD (Fig. 6). Thruout this paper I shall use the nomen- 

 clature of Conklin as the clearest and the most applicable to the situation 

 as found in Haminea. 



Of the two cells produced by the first cleavage, the smaller one (CD) 

 is the posterior; the larger (AB) the anterior. In the inequality of the 

 first cleavage Haminea agrees with Umbrella (Heymons 1893) and 

 Philine (Guiart 1901); Crepidula (Conklin 1897), Fiona (Casteel 1901), 

 Tethys (Viguier 198), and Planorbis (Holmes 1900), have on the con- 

 trary an equal first cleavage. Following the cleavage the cells round into 

 two nearly perfect spheres, and the two nuclei return to the resting condi- 

 tion, lying on a perpendicular bisector of the first cleavage furrow when 

 the animal pole is directly up (Fig. 6). This position they appear to keep 

 during the subsequent flattening against each other of the blastomeres, 

 and in fact until the formation of the spindles for the next cleavage 

 (Fig. 7). Conklin (1897) finds in Crepidula that "at the close of the 

 first cleavage the nuclei, asters, and archoplasmic bodies lie op))osite each 

 other in the blastomeres ; but as soon as the blastomeres begin to flatten 

 against each other, and the whole egg assumes a more conii)act form, all 

 these structures move in the direction of a clock's hands. This movement 



