32 WHITMAN. [ Vou. II. 
cytoplasm. The hypothesis of reciprocal action is not incom- 
patible with the opinion that the conditions of this action are 
furnished, in the first instance, if not continuously, by changes 
of a chemical or molecular nature, which arise quite indepen- 
dently, either in one factor alone, or in both. The source of 
the initiatory impulse would still be an open question. Our 
knowledge of the phenomena above designated is too incom- 
plete to furnish a key to the solution of this problem. For 
the purpose we have in view, it will be sufficient, therefore, to 
refer to a few of the more important examples, in which each 
factor may be supposed to play a more or less important part, 
deferring the discussion of the main question until we come to 
consider phenomena of a more decisive nature. 
Following the penetration of the spermatozoon into the 
ovum, various forms of contraction in the vitellus have been 
observed; and these are generally regarded as the effect of 
impulses generated by the spermatic element. The usual 
sequence of events certainly accords very well with this view, 
but there are one or two facts which should make us hesitate to 
accept it. In some aquatic animals, in which the sexual cells 
unite before ovipositing, the time of these contractions bears 
no constant relation to the time of union, but does bear such a 
relation to the time of contact of the egg with water, whether 
this contact be brought about artificially or in the natural 
course of events. Still another cogent reason for not ascribing 
these contractions to the zwdependent action of the male pro- 
nucleus is found in the fact that similar, though more sluggish, 
movements may, in some well ascertained cases, be induced by 
placing unfertilized eggs in water. The most general of these 
movements is the flattening of the pole, and the gradual con- 
traction of the whole vitelline sphere, resulting in the forma- 
tion of a perivitelline space. 
The Constriction Attending the Exit of the Polar Globules. — 
The flattening of the pole is attended, or followed, in some cases, 
with a very remarkable constriction, which, beginning in the 
equatorial zone, travels towards the animal pole, finishing up with 
a nipple-like protuberance, from which the first polar globule is 
expelled. The exit of the second polar globule is sometimes pre- 
ceded by a similar but weaker constriction. This constriction 
has been observed (No. 12, p. 18) in the eggs of different species 
