14 LILLIE. [Vol. X. 



also noticed that " eggs from the same animal show similar 

 variations in cleavage" ; he concludes that "such a tendency 

 to vary may become hereditary." 



The first generation of ectomeres occupy a position in relation 

 to the macromeres which might be supposed to be reached by 

 their rotation after formation through an angle of 45° in the 

 direction of the hands of a watch. This arrangement is 

 obviously well adapted to economize space ; if it were produced 

 by an actual rotation of the cells after their formation there 

 would be no difficulty in understanding it. But the fact that 

 the cleavage spindles are oblique, even before the lobing of the 

 cytoplasm, shows that the process is not a purely mechanical 

 one, in the sense that it is produced by the mutual pressure of 

 the blastomeres. This manner of division is characteristic of 

 the ovum of molluscs (cephalopods excepted), annelids and 

 polyclads, Selenka (No. 58^) was one of the first authors to 

 describe it. Speaking of the four ectomeres, Selenka says 

 that they are budded off from the four macromeres "im 

 Sinne einer laeotropen oder X-Spirale." Blochmann (No. 35), 

 who was the next to describe this type of cleavage, in a proso- 

 branch, Neritina, used a different method of expression ; he said 

 that, looked at from the animal pole, the first generation of ecto- 

 meres during their formation underwent a shifting " im Sinne 

 des Uhrzeigers," and the second generation " in der Uhrzeiger- 

 bewegung entgegengesetzten Sinne." Lang (No. 53) used the 

 same form of expression as Selenka ; but neither of these 

 authors has made use of the term spiral to distinguish a special 

 type of cleavage. Wilson, on the other hand, has characterized 

 this form of cleavage as the "spiral" type in distinction from 

 his " radial " and "bilateral " types. Heymons (No. 47) seems to 

 prefer Blochmann's comparison with the hands of a watch, for 

 he simply refers to the other expression as " sog. spiralige." 

 Kofoid (No. 49«) uses the term spiral in the same way as Wilson. 

 It seems, then, that there have been two ways of describing 

 this form of cleavage: first, by the use of the term spiral, and 

 second, as a rotation. 



Wilson (No. 65, p. 600) derives the " spiral " from the " radial " 

 form of cleavage by " a twisting of the radii, as it were, the 



