No. 2.] THE DEVELOPMENT OF THE NEWT 323 



(Fig. 33F) indicate sufficiently well that the early stages of 

 cleavage do not, in this case, rigidly mark out the future de- 

 velopment of the embryo. The course of embryonic develop- 

 ment in some animals is not dependent on the position or 

 indeed the existence of all the early cleavage nuclei. We are 

 consequently led to the conclusion that the parts are not essential 

 to the integrity of the whole, that indeed the whole may be de- 

 prived of some of its parts and yet remain the whole. There is 

 no qualitative sorting out of nuclear substance in the early cleav- 

 age stages ; on the contrary a large proportion of nuclear sub- 

 stance can be displaced or even sacrificed without impairing the 

 individuality of the embryo, or rendering it incapable of normal 

 development. Wilson (-92) in his brilliant research upon the cell- 

 lineage of Nereis, reached the conclusion that " Blastomeres 

 having precisely the same mode of origin and precisely the 

 same spatial relations to the rest of the embryo are by no means 

 necessarily equivalent, either physiologically or morphologically, 

 and the early cleavage-stages in themselves have little morpho- 

 logical value " (p. 455). Such a conclusion, drawn from annelid 

 cleavage where the cell formation follows definite lines and the 

 differentiation of important organs is strikingly precocious, is 

 certainly applicable with added stringency to the other extreme 

 of amphibian cleavage where the initial cleavage stages exhibit 

 as a rule chaotic irregularities and variations. 



Turning now to quite a different question, let us ask why 

 the first cleavage plane should fall as it does in any individual 

 case. Why should the plane pass through any particular me- 

 ridian rather than through any other } 



In the newt the most interesting fact about the first cleavage 

 plane is its determinateness. It cuts the ^gg invariably, so 

 far as my observations go, at right angles to the long axis of the 

 enveloping capsule. In removing the ^gg from the capsule I 

 have unfortunately not been able to preserve the orientation of 

 the Qgg, and so have not succeeded in determining whether the 

 (tgg when freed from the capsule still cleaves as it would have 

 done inside. This constancy of direction of the first cleavage 

 plane in the encapsuled Qgg puts an obstacle in the way of ac- 

 cepting for the newt the view that Roux has advanced for the 



