I 1 2 MCMURRICH. [Vol. XI. 



is to take part in tJic formation of the blastoderm, and this segre- 

 gation supervenes not in aceordance zvitk any previous loeation of 

 a nueleus, but independently. I do not of course mean to assert 

 that the nuclei may not possess a coordinating or even a trophic 

 action upon the cytoplasm, but that they are directly respon- 

 sible for the segregation or concentration seems to me an un- 

 warranted assumption. The phenomenon stands apparently in 

 relation to the growth of the entire organism rather than to that 

 of a part of it, and is an instance from the animal kingdom in- 

 dicating the distinction so forcibly pointed out by Sachs ('87) 

 as existing between growth and cell division. 



It has already been pointed out (see p. 74) that in the four- 

 celled stage of Jaera there is an arrangement of the spherules, 

 apparently comparable with that found in holoblastic ova which 

 undergo a " spiral " cleavage. In later stages, however, the 

 cleavage departs widely from the typical mode of progress of 

 the spiral method, probably in harmony with the precocious 

 differentiation which has already been shown to exist. The 

 Isopod segmentation agrees strictly with none of the three types 

 of cleavage defined by Wilson ('93), though it bears most resem- 

 blance, in early stages at least, to the " spiral " method, and the 

 peculiarities of later stages are to be ascribed to the same 

 causes as Wilson proposes in explanation of the bilateral type. 

 In his earlier discussion of the forms of cleavage ('92) Wilson 

 held that the spiral type of cleavage was determined by the 

 spherules having a tendency to arrange themselves along the 

 lines of least resistance, assuming therefore a purely mechani- 

 cal cause acting from the exterior as an explanation. In his 

 later paper, however, he modifies this extreme view, stating his 

 opinion that " eleavage forms are not determined by mecJianieal 

 conditions alone." Assuming that by " mechanical conditions " 

 he means conditions extrinsic to the ovum, I believe that in 

 Jaera we have practically a demonstration of the correctness of 

 this view. 



Let us briefly recall the rearrangement of the protoplasm 

 which obtains \w Jaera. Completely surrounding the exterior 

 of the ovum is a thin layer of peripheral protoplasm, correspond- 

 ing to the " Kcimhautblastcm " of the Qgg of Insects, and 



