MUSEUM OF COMPARATIVE ZOOLOGY. 517 



ous fishes before fecundation by Reichert, Oellacher, Van Bambeke fVC", 

 pp. 1-12), and many others. In the less conspicuous form of a 

 gradual migration, it may be as nearly universal as the occurrence of 

 polar globules. Even in Limax, where no evidences of irregular pseu- 

 dopodal projections have been seen,* the primary pole becomes clearer 

 after the formation of the maturation spindle, and with the third act 

 of cleavage the antithesis between the two hemispheres reaches a 

 maximum. 



Whitman has concluded, from the position of the nuclear structure 

 near the primary pole, that the pseudopodal phenomena in Hydra, as 

 well as the " Faltenkranz " of amphibian eggs, which he considers due 

 to the same cause, are to be classed with the radial arrangements of the 

 yolk during cleavage, and consequently to be referred to nuclear influ- 

 ence ; just as the place where the segmentation furrow is first to appear 

 depends on the location of the amphiaster, being always manifest earlier 

 on the side of the yolk toward which the segmentation aster is most nearly 

 approximated. As regards cleavage, I believe he is right in saying that 

 the same relation is probably true in all cases, — at least, I have met 

 with only two or three instances which seem to conflict with that rule.f 

 I also agree with him in believing the phenomena have a causal relation. 

 He thinks the pseudopodia are most pronounced, and the cleavage first 

 expressed, at the primary pole, because of nuclear influence. But he 

 does not explain why the nucleus has this eccentric position, — why it 

 is near this pole rather than some other point of the periphery. The 

 nucleus appears ultimately to assume a position of equilibrium, not with 

 regard to the whole mass of the egg, but in respect to its active con- 

 stituents. Is not, then, this peculiarity ultimately, though indirectly, 



* P. S. — The later observation reported on p. 180, foot-note, serves to show the 

 inconstancy of the pseudopodal phenomena. I believe they can be considered only 

 as special manifestations, whereas the polar concentration of active substance is oi 

 more fundamental significance. 



t Kowalevsky ('75, Taf. XXXVIH. Fig. 14) represents a stage in the segmenta- 

 tion of Pyrosoma where the furrow seems to advance first from the side op})osite that 

 occupied by the nuclear figure. 



To judge from the figure given by L. Agassiz ('62, PI. XXXI. Figs. 2, S**), one 

 might expect to find the first cleavage furrow in Laomedea amphora appearing earlier 

 on the side opposite the peduncle, since in Fig. 2 the germinative vesicle lies nearest 

 that side of the egg. The natural though not necessary inference is that this side 

 corresponds to the primary pole. But it is stated (p. 313) that "the process of seg- 

 mentation .... commences by forming a farrow across the yolk on that side which 

 lies next to the peduncle of the medusa." 



See also the account (p. 419) of Giard's statement for Salmacina. 



