JENNINGS: DEVELOPMENT OF ASPLANCHNA HERRICKL. 83 
Rhabditis nigrovenosa: “Ich stelle mir die Mechanik des Gastrulations- 
vorganges so vor, dass die Ektodermzellen nach ihrer Theilung sich 
abflachen und in Folge dessen ausbreiten; dabei schioben sie die 
Mesodermzellen über die Entodermzellen herüber. Es kann dies um so 
eher geschenen, da die Mesodermzellen zum Zweck der Theilung sich 
kugelig zusammengezogen und dabei an die Oberflüche des einschichtigen 
Epithels emporgehoben haben." 
On the other hand, zur Strassen ('95 and '96), in his careful studies 
on the early development of another nematode, Ascaris mogalocephala, 
camo 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 Roux (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 significanoe for the movements 
which take place. 
3. GENERAL CONSIDERATIONS. 
T shall next take up certain general considerations upon the mechan- 
ies of cleavage aud 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 Asplanehna at the four-cell stage might be compared to 
the egg of an echinoderm, with the exeeption 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 ferm and size of 
the blastomeres, 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 
