No. 3.] THE CLEAVAGE OF AMPHIBIAN OVA. 409 
Each egg, as a rule, possesses an individual rhythm of 
cell-division, and the time intervals between the different sets 
of furrows are substantially the same in the same egg. There 
is, however, considerable variation between these rhythms in 
different eggs. We have, for example, observed the fourth set 
of furrows in an Amblystoma egg follow the third set in one 
hour and thirty minutes, while in another egg from the same 
mother the fourth set appeared only after an interval of two 
hours and forty-five minutes. 
Making all due allowance, however, for this asynchronism, 
one fact stands out prominently in the table above. This is 
the fact that the inter-cleavage periods in the Urodelan eggs 
are much longer than the corresponding periods in the Anuran 
eggs. This difference does not depend upon the size of the 
egg. The small egg of the newt and the large egg of Ambly- 
stoma have practically the same rapidity of cell-division. The 
ege of Rana, on the other hand, which is considerably larger 
than the egg of the newt, divides much more speedily than 
the latter. We see no escape, therefore, from the conclusion 
that in this instance the rapidity of cell-division depends upon 
the innate and inherited tendencies of the cytoplasm and 
nucleus, rather than upon the size of the ovum. This view 
receives confirmation from the fact that the whole course of 
Urodelan and Anuran development is marked by a similar 
disparity intime. The formation and closure of the blastopore, 
the formation and closure of the neural folds, all take place 
more expeditiously in the Anuran than in the Urodelan 
embryos, while the Anuran tadpole frees itself from its gela- 
tinous envelopes at a time when the Urodelan tadpole of the 
same age is destined to remain imprisoned for some ten days. 
The whole course of Urodelan development, therefore, from 
the entrance of the sperm (see Jordan, '93) to the release of 
the larva, is slower than that of the Anuran. This difference 
in rapidity of cell-division extends, as we have shown, to the 
earliest cleavage stages, and is foreshadowed in the pre-cleav- 
age phenomena. 
The quantity of yolk, however, exerts an unmistakable influ- 
ence upon the speed with which furrows cut their way to the 
