308 bulletin: museum of comparative zoology. 



That the cleavage centrosomes arise de novo from tlie cytoplasm of the 

 egg has been maintainetl by the following writers : Foot ('97) in Allolo- 

 bophora fcetida, Child ('98) in Arenicola, Lillie ('97, :0l) in Unio, Foot 

 and Strobell (:01, p. 606) in Allolobophora foetida, and Smallwood (:0l) 

 in Bulla. Conklin (:0l) has shown for Crepidula that the cleavage 

 centrosomes arise, one from the male sphere-substance, the other from 

 the female sphere-substance; while Wilson (:0l'') has proved conclusively 

 that centrosomes may arise in connection with the female pronucleus. 

 In describing the effects of ether on Arbacia eggs he says (lOl'', p. 360) 

 in regard to the union of the male with the female pronucleus, " As a 

 rule the nuclei remain distinct, separately undergoing a highly interest- 

 ing series of changes. The most striking fact is that while the sperm- 

 aster often gives rise to a perfect and symmetrical bipolar division-figure, 

 thq egg nucleus, in a great number of cases, produces a monaster which 

 seems at first incapable of resolving itself into a bipolar figure." He 

 adds (p. 362), " Although the egg-monaster does not at first divide, it 

 may do so in later stages." 



The relation of the two pronuclei to the cleavage asters at their first 

 appearance indicates a great want of agreement among different animals, 

 although the conditions remain constant for each species. In Pleuro- 

 phyllidia (MacFarland, '97) the cleavage asters arise in almost any rela- 

 tion to each other and to the two pronuclei ; while in Toxopneustes 

 (Wilson, '95), in Chtetopterus (Mead, '98), in Cerebratulus (Coe, '99), 

 and other invertebrates the new cleavage asters bear a very definite rela- 

 tion to each other and to the two pronuclei. Between these two extremes 

 are found intermediate conditions in which the new cleavage asters have 

 no fixed relation to each other, but a very definite one to the pronuclei, 

 inasmuch as one of them is associated with each pronucleus. Such 

 an intermediate condition has been observed by Lillie (:0l) in Unio, 

 by Conklin (:0l) in Crepidula, and by Smallwood (:0l) in Bulla 

 (Haminea). 



Linville (:00) figures for Linma?a elodes and Limax maximus condi- 

 tions which show that one of the new cleavage asters may be found as- 

 sociated with each pronucleus. Linville believes this to represent a 

 " temporary and incidental " state. In two of the three eggs reproduced 

 the asters are the youngest ones represented by him. It is entirely con- 

 ceivable that the asters may become united to each other, and the asso- 

 ciation with each pronucleus may be a temporary one ; but it does not 

 seem to me to be incidental in Haminea, because, from the conditions in 

 a large number of eggs examined, it is clearly unusual for both the new 



