50 BULLETIN: MUSEUM OF COMPARATIVE ZOÖLOGY. 
of the spindles, which exists before there is any constriction of the cyto- 
plasm. As in the case of the first cleavage plane, the constriction pro- 
gresses most rapidly from the animal pole. Toward the close of the 
process of constriction the daughter cells are united by only a bridge of 
protoplasm, which is nearer the vegetative pole. Figure 16 (Plate 11.), 
from a preparation of a slightly older stage, shows a similar bridge of 
protoplasm, but it is much nearer tho animal pole. 
The period of constriction is followed by a stage similar to that 
described for the two blastomeres, in which each of the four blastomeres 
assumes a spherical contour and stands out sharply and distinctly from 
its neighbors. This phase soon passes, and within half an hour the egg 
has assumed the condition of Figure 9 (Plate L). This is the typical 
four-cell stage of the spiral type of cleavage, and therefore merits further 
description. (See diagram of this stage on page 52, Figure A.) The 
four cells, A, B, C, D, occupy the left anterior, right anterior, right 
posterior, aud left posterior quadrants respectively. Each cell presents 
to the exterior a rounded, convex surface, and upon its inner side 
has three facets of contact, — the first and third with the cells of the 
adjacent quadrants, the second with the cell of the diagonally opposite 
quadrant. This last facet is triangular in shape, with its base at one 
pole and apex near the centre of the egg. The vertical axis of the egg 
lies in the planes of these central triangular facets. The bases of tho 
central facets coincide with the well known cross furrows of the animal 
and vegetative poles of tho egg (compare Plate II. Fig. 17). The cross 
furrow of the animal pole lies between the cells 4 and C, and extends 
from D to B, that of the vegetative pole lies between B and D, and 
extends from A to C. Thus by this mutual adaptation of the cells to 
one another, the spheroidal form of the egg as a whole is, in a degree, 
again restored, and here, as in the two-cell stage, persists during tho 
period of “nuclear quiescence.” I have referred to the condition in 
Limax as “typical.” I mean that the conditions are simple, and that 
the modifying influence due to the presence of a large amount of yolk, 
and its equal or unequal distribution among the four blastomeres, is 
absent. 
A comparison of the conditions presented here (Plate I. Fig. 9, Plate II. 
Fig. 17, and Fig. A, p. 52) with the same stage in other animals shows 
how profound the modifications are. In Limax the dorsal and ventral 
cross furrows are approximately equal in length, and as seen from the 
animal pole lie nearly at right angles to each other. In Nereis (Wilson 
92) the dorsal furrow is largely obliterated, the four blastomeres almost 
