JEXXINGS: DEYELOPMEXT OF ASPLAXCHXA HERRIGKII. 83 



Rhabditis nigrovenosa: " Ich stelle mir die Mechauik des Gastrulations- 

 vorganges so vor, dass die Ektodermzellen nach ibrer Tbeilung sich 

 abflacben und in Fulge desseu ausbreiten ; dabei schieben sie die 

 Mesodermzellen ilber die Entodermzelleu heriiber. Es kann dies urn so 

 eher gescheuen, da die Mesodermzelleu zum Zweck der Tbeilung sich 

 kiigelig zusammengezogen und dabei an die Oberflache des einschichtigeu 

 Epithels emporgehoben haben." 



On the other hand, zur Strassen ('95 and '9G), in his careful studies 

 on the early development of another nematode, Ascai'is megalocephala, 

 came to an entirely different conception of the factors at work in the 

 displacement and extension of the ectoderm cells. Zur Strassen holds 

 that four cells of common origin constitute an " elementary mechanism," 

 the two pairs of which attract each other in such a way as to bring about 

 the movements which actually occur. The interaction is thought of as 

 something having the nature of the "cytotropism " of Eoux ('95). It 

 is not necessary to discuss the matter further here, since there is no 

 occasion to call in any such action in the gastrulation of Asplanchna. 

 Doubtless the inter-attraction of cells exists in the rotifer, as elsewhere, 

 but it seems to have no determining significance for the movements 

 which take place. 



3. General Coxsideratioxs. 



I shall next take up certain general considerations upon the mechan- 

 ics of cleavage and development, which do not naturally fall under the 

 discussion of any of the theories above considered, together with a 

 general review of the results gained. 



The egg of Asplanchna at the four-cell stage might be compared to 

 the egg of an echiuoderm, with the exception that one of the four 

 blastomeres is much enlarged, and of a different form from the others. 

 "What variation in the cleavage will be induced by the differences in 

 form ? 



As we have seen, the form of the cell in Asplanchna does- not affect 

 the direction of the spindle at cleavage. Indeed, the most character- 

 istic feature of cleavage in Asplanchna is difference in form and size of 

 the hlastomeres, coupled with identity in the direction of spindles, in cells 

 having the same general relations to the axes of the embryo. 



In a closer consideration of the factors determining the position of 

 spindles, it is evident from the phenomena described that the question 

 is not a simple one, but must be resolved into (1) what determines the 

 direction of separation of the asters ; (2) what determines the position 



