THE SPIRAL TYPE OF CLEAVAGE. 255 



division balancing others and aiding in the accomplishment of 

 the "desired" end, vi::., the concrescence of the plate and the 

 formation of the " growing tip " ! Moreover, the posterior move- 

 ment of the descendants of the first somatoblast which is ini- 

 tiated by the third division — the first symmetrical division — 

 begins the covering over of the mesoblast. The mesoblast 

 divides bilaterally at its first division, for it is formed in such a 

 position that it must be covered over by the somatic plate and 

 thus forced into the interior of the ^gg, and its fate is to fur- 

 nish the double " Anlage " for the mesoderm bands. I think 

 these examples are sufficient to show how exactly the appear- 

 ance of the bilateral cleavages is timed for the accomplishment 

 of the purpose of development in the most perfect manner. 

 And, to my mind at least, they furnish ample explanation of 

 the fact that the bilateral cleavage does not appear in the 

 earlier stages. 



It may be urged as an objection to this view that in various 

 forms {e.g., ascidians) the first cleavages are bilateral, and the 

 distribution of the material is accomplished equally well. This, 

 of course, is perfectly true, but there is no evidence that the 

 bilateral form of cleavage has displaced a spiral form in this 

 case. In the annelids and mollusks the spiral form of cleavage 

 is undoubtedly older than the bilateral form, and has been in 

 part displaced by it, the change occurring first in later stages, 

 and proceeding to the earlier. The point made here is simply 

 that there, is no adequate cause for the displacement of the 

 spiral by the bilateral form of cleavage in stages earlier than 

 those at which the latter now appears, unless of course the 

 spiral period could be eliminated at one blow, as it were, and a 

 bilateral cleavage could begin at once. This may perhaps be 

 a more complete modification, but there are no indications dis- 

 cernible of its appearance, unless, indeed, the cleavage of such 

 forms as Teredo and Cyclas is to be regarded as such. The 

 spiral form of cleavage seems to serve perfectly in plotting out 

 the material of the Qgg in these forms. 



In general, the directio7i of each cell division in morphogenetic 

 cleavage plays a perfectly definite role in the accomplisJiment of 

 the morphogenesis of the species concerned. Bilaterally symmet- 



