52 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
place in a right spiral, whereas that of the third presented the character 
of a left spiral. Applying the system of nomenclature which I have 
adopted to the derivatives of the third generation, we find that cells of 
the lower quartet will be designated by the exponent 4.1, and the upper 
by 4.2. It will be convenient in the further discussion of quartets to 
refer to them simply by their exponents, without reference to the indi- 
vidual cells of which they are composed. It will be seen from Figures 
17 and 19 (Plate II.) that the dorsal and ventral cross furrows at the 
close of this stage do not lie at right angles to each other, as they did at 
the end of the four-cell stage, but that they cross each other at an angle 
as is represented by the shifting of the cells to 
as much less than 90° 
5?, as seen in the 
produce the spiral, i. e. they now cross at about 45%, 
accompanying diagrams (Figures A and D). 
FIGURE B. 
FIGURE A. 
Figure A is a diagrammatic representation of the four-cell stage of 
Limax as seen from the animal pole, showing dorsal and ventral cross 
furrows. Figure B is the same of the eight-cell stage. 
This condition is not quite realized in Figure 21 (Plate TIT.), owing to 
the near proximity of the succeeding division, which restores the cross 
furrows approximately to the conditions of the four-cell stage. Thus, in 
the typical cight-cell stage of Limax the cross furrows correspond to 
those of the same stage of Nereis (Wilson 792, Plate XIV. Fig. 11). 
In Umbrella likewise (Heymons '93, Taf. XIV. Fig. 4) the dorsal and 
ventral furrows are oblique to each other, crossing at about 45?, but 
differing in this important respect from the ‘furrows of Limax and Nercis, 
that they are in this caso formed by the juxtaposition of the cells of 
quadrants B and D at both poles, whereas in Limax and Nereis the 
s of these quadrants, the dorsal fur- 
ventral furrow only is formed by cell 
The furrows 
row being formed by a** and c*?, as is shown in Figure D. 
