626 



GRIFFIN. 



[Vol. XV. 



shown by both their origin and their fate ; for each sphere is 

 really a loop representing the approximated halves of two adja- 

 cent quarters, as shown in the accompanying diagram (HI). 



In both figures the horizontal and vertical lines represent the 

 division planes, and the included portions (similarly lettered in 

 the figures) homologous parts. The horizontal lines in both 

 coincide with the long axis of the spireme and the transverse 

 axis of the spindle (equation division), while the vertical line 



Fig. III. — ..4 , tetrad of the Copepod type ; .5, spurious tetrad (cross-form) of 

 Thalassema or Zirpkaea . 



is transverse to the spireme-axis or parallel to the spindle-axis 

 (reducing division). In A whole spheres are thus separated, in 

 B each sphere (loop) is halved. 



Should the tetrad shown at B be shifted 45 degrees so as to 

 make the division planes separate entire spheres, as is the case 



in A, the formula of the tetrad would be not 



-T' as in the first 

 



case, but 



a a \a d 

 2 22 2 



ad 

 2 2 



2 2 



There seems to be no theoretical 



reason why such a mode of division may not occur in nature, 

 although up to the present time none has been definitely ob- 

 served. If the " object " of reducing divisions be, as Weismann 

 supposes, to provide a source of variation, there would be 

 obviously an advantage in the above, since the number of 

 possible combinations is greater in the second case than in 

 the first. The Elasmobranch rings figured by Moore ('95) 

 show four thickenings, which, however, are halved like the 

 loops in Zirphaea, but unfortunately, as nothing is said about 



