■18 Pub. Paget Sound Biol. Sta. Vol, 2, No. 34. 



of the nuclei and asters takes place invariably in the same direction^ and 

 it must therefore have been predetermined during, and perhaps before, 

 the first cleavage. In Crepidula the first spindle does not seem to indicate 

 any such rotation, though it is exceedingly significant to note that Warneck 

 (1850) in the case of Limax and Fol (1875) in Cymbulia found that the 

 first cleavage was oblique to the line of elongation of the egg. Kofoid 

 (1895), however, in his recent careful work found no evidence in favor 

 of Warneck's account." 



In the preserved material in Haminea the writer finds no indication 

 of any such rotation of the nuclear structures after the first cleavage is 

 completed ; it is, however, possible that in the living egg some such move- 

 ment might be observed. There is no evidence of the occurrence of this 

 phenomenon in Fiona Casteel (1904); Heymons (1893) and Viguier 

 (1898) make no mention of its occurrence in Umbrella and Teihys. Defi- 

 nite rotation of the first nuclear spindle in Haminea gives further support 

 to the conclusion reached by Conklin that "in all cases in which the second 

 cleavage is laeotropic the first is dexiotropic, and that the initial cause 

 of the spiral cleavages is not to be found in the direction of the nuclear 

 spindles but rather in the structure of the unsegmented egg itself." 



SECOND CLEAVAGE 



As the spindles for the second cleavage form at right angles to the 

 plane of the first, the two cells flatten against each other and each elon- 

 gates {Fig. 8). The spindles do not lie in the same plane, but are slightly 

 mclined, so that when seen from the side they appear to cross each other 

 slightly. This cleavage consists in reality of two rapidly succeeding cleav- 

 ages (Fig. 8), the division of the smaller, posterior blastomere (CD) 

 slightly preceding that of the larger, anterior (AB). The furrows of the 

 second cleavage, like those of the first, proceed from the animal to the 

 vegetal pole, dividing each of the two cells equally. These furrows do not 

 meet in a point at either pole, but at the upper pole each proceeds from 

 a point a little to the left of the polar bodies when observed from the center 

 uf the egg; at the lower pole each furrow meets the first cleavage furrow 

 at a point still further removed to the left (Fig. 9). Thus the cleavage is 

 diagonal and not meridional, and clearly indicates laeotropic sijiraiity, in 

 that from B and D, two new cells, A and C, are cut off in an anti-clockwise 

 spiral. As is the rule for all known cases of dexiotropic cleavage, the 

 lower, or ventral, cross-furrow, formed between the points of juncture of 

 the first and second cleavage, bends to the right when viewed from the 

 animal pole along the first cleavage furrow. The upper cross furrow is 

 parallel to the lower, but much shorter, at times being almost indistin- 



