50 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



of the spindles, which exists before there is any constriction of the cyto- 

 phism. 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 danghter cells are united by only a bridge of 

 protoplasm, wliich is nearer the vegetative pole. Figure 16 (Plate II.), 

 from a preparation of a slightly older stage, shows a similar bridge of 

 protoplasm, but it is much nearer the 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 I.). 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, and left posterior quadrants respectively. Each cell presents 

 to the exterior a rounded, convex surface, and upon its inner side 

 has thi'ee 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 the 

 central facets coincide with the well known cross furrows of the animal 

 and vegetative poles of the egg (compare Plate II. Fig. 17). The cross 

 furrow of the animal pole lies between the cells A and C, and extends 

 from Z> to B, that of the vegetative pole lies between B and I), 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 wiiole is, in a degree, 

 again restored, and here, as in the two-cell stage, persists during the 

 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 tlie 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 



