62 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
readily group themselves into quartets. A median deep-lying mesoderm 
cell is present, as in the egg last described, but the relations of the cells of 
the quartet of the vegetative pole to this cell and to one another are differ- 
ent from those of the other eges, in the following respect: the cells which 
meet in the ventral cross furrow are a"! and c^, instead of d™ and 07, as 
in the other two eggs figured. Repeated trials failed to give any other so- 
lution which would accord with the conditions in the other parts of the egg. 
The juxtaposition of the quadrants a and ¢ occurs normally in tho reversed 
type of cleavage (see Figs. C and D, p. 53), and the possibility is at once 
suggested that this egg may have had reversed rather than unreversed or 
normal cleavage. Other parts of the egg, however, furnish no corroborative 
evidence, and the suggestion must be dismissed. I believe, then, that 
owing to some cause, mechanical or other, a change in the normal rela- 
tions of the cells of the quartet to one another has been brought about. 
This has naturally raised the question as to the constancy of the cross 
furrows, upon which the orientation of the egg so largely depends. 
This case in Limax is not an isolated one, for in Nereis, where, as has 
already been pointed out, the dorsal furrow is formed in the early stages 
of cleavage by the apposition of the quadrants «a and c, we find this 
normal arrangement disturbed in one instance, the furrows being formed 
by the quadrants b and d (Wilson ’92, Plate XIV. Fig. 19, p. 390). In 
the later stages, i. e. after the cells of the seventh generation appear at 
the animal pole, the dorsal eross furrow is normally formed by the 
apposition of b and d, but in one case (Wilson /. c., Plate XVI. Fig. 35) 
we find this arrangement disturbed, the cross furrow being formed by a 
and c. This disturbance is also accompanied by the mitotic conditions 
of neighboring cells. 
Another case occurs in Neritina (Blochmann '81, Taf. VIT. Figs. 51, 
53, 56), in which two eggs — one a thirty-six-, the other a forty-cell 
stage — present cross furrows formed by the apposition of different pairs 
of quadrants. There is not here, as in Nereis, an intervening mitosis to 
explain the disturbance of the customary order, 
In Lang's (85) Taf. 34, Figs. 14, 15, we find a similar transposition 
from the usual arrangement of the apical quartet, accompanied in this 
case by mitosis in adjoining cells. In view of these cases it seems not 
improbable that there has been in this Limax egg a disarrangement 
of the normal condition at the vegetative pole, as a result perhaps of the 
recent divisions at that pole, the collapse of the cleavage cavity, or some 
other mechanical disturbance. 
It seems almost certain that the primary mesoderm cell, d^? (M ), is 
