62 BULLETIN: MUSEUM OF COMPAKATIVE 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 eggs, in the following respect : the cells which 

 meet in the ventral cross farrow are a''^^ and c'^\ instead of d^-^ and 6'' \ as 

 in the other two eggs figured. Repeated trials failed to give any other so- 

 lution which would accord with the conditions in the otlier pai-ts of the egg. 

 The juxtaposition of the quadrants a and c occurs normally in the 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 

 Aormal 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 N^ereis, '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 cross furrow is normally formed by the 

 apposition of b and d, but in one case (Wilson l. c., Plate XVI. Fig. 35) 

 we find this arrangement disturbed, the cross fiuTow 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. VII. 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 

 fi'om 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^-^ {^^), is 



