52 



BULLETIN : MUSEUM OP 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 much less than 90° as is represented by the shifting of the cells to 

 produce the spiral, i. e. they now cross at about 45°, as seen in the 

 accompanying diagrams (Figures A and B). 



Figure A. 



Figure B. 



Figure A is a diagrammatic representation of the four-cell stage of 

 Li max 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 III.), 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 eight-cell stage of Limax the cross fuiTows correspond to 

 those of the same stage of Nereis (Wilson '92, Plate XIV. Fig. 11). 

 In Umbrella likewise (Heyraons '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 Nereis, 

 that they are in this case formed by the juxtaposition of the cells of 

 quadrants B and D at both poles, whereas in I^imax and Nereis the 

 ventral furrow only is formed by cells of these quadrants, the dorsal fur- 

 row being formed by a*-^ and c*-^, as is shown in Figure B. The furrows 



