54 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



the egg, but there seems to be no acceleration of the division in the 

 mesoderm-prodncing quadrant D over the quadrant C in this egg. 



The division, as has been before stated, is in individual planes oblique 

 to each other, and not in a common equatorial plane such as occurs in 

 radial division. In Limax the cells of the two quartets 4.1 and 4.2 are 

 of unequal size, the inequality being almost as great as in Nereis ; this 

 will be seen on a comparison of my Figure 19 with Wilson's ('92) Plate 

 XIII. Fig. 10. We may therefore distinguish the components of the 

 larger quartet as macromeres, and the smaller as raicromeres of the first 

 set. This difference in size, so marked at this period, persists in Nereis 

 to a very late stage of development, but in Limax it is practically oblit- 

 erated at the division leading to the next generation. Hence it is that 

 a system of nomenclature based on these distinctions loses its significance 

 when applied to the approximately equal cleavage of Limax. 



Fifth Generation. Sixteen Cells. 



With the formation of the two quartets of the fourth generation it has 

 become no longer possible to designate a single cleavage furrow as pro- 

 ducing the next generation. I shall therefore discuss the cleavage, from 

 this time on, from the standpoint of the successiVe cleavage of quartets. 



Division of Quartet \.\, forming 5.1 and 5.2. 



Plate III. Figs. 30, 31. 



The basal and larger quartet of the eight-cell stage is seen in a 

 mitotic condition in Plate III. Figs. 20 and 21. Here, as in the previous 

 division, the spindles are not vertical, but much inclined ; this time, how- 

 ever, the upper asters of each spindle lie to the left,^ and not to the right, 

 of the lower ones (Fig. 20). The division of the chromatin elements 

 has already taken place in the spindles of this quartet (best seen in 

 Fig. 21), and the conditions of the completed mitosis can readily be 

 inferred from the figures. If we view the egg from tlie animal pole, and 

 pass from the lower derivative to the upper, we move in a direction 

 opposite to that of the hands of a clock, i. e. this division takes place in 

 a left spiral. The division of this quartet (4.1) is almost equal (Plate 

 HI. Fig. 22), the basal derivatives (5.1) being but slightly larger than 

 the upper ones (5.2). In this respect Limax differs from all the yolk- 

 laden forms, — Neritina, Planorbis, Umbrella, and Nereis, — where the 



1 Cf. Kofoid '94, p. 180, for explanation of the use of right and left. 



