54 BULLETIN: MUSEUM OF COMPARATIVE ZOÖLOGY. 
the egg, but thore seems to be no acceleration of the division in tho 
mesoderm-produeing 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 
radialdivision. 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 
XIIL Fig. 10. We may therefore distinguish the components of the 
larger quartet as macromeres, and the smaller as micromeres 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 4.1, forming 5.1 and 5.2. 
Plate III. Figs. 20, 21. 
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 /eft,! 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 scen in 
Fig. 21), and the conditions of the completed mitosis cam readily be 
inferred from the figures. If we view the egg from the animal pole, aud 
pass from the lower derivative to tho upper, wo movo 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 
III. 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 '04, p. 180, for explanation of the use of right and left. 
