JENNINGS: DEVELOPMENT OF ASPLANCHNA HEEPtlCKII. 69 



But mere increase of the number of cells without change of form is 

 not the only object which can be brought about by cell division, and 

 when other purposes are to be accomplished the cells are so constituted 

 as to react in a different manner. Thus, I have shown that in Asplanchna 

 divisions of cells with spindles in the short axis is the method by which 

 is brought about the continued extension of cells in one direction, with 

 consequent gastrulation and a later invagination of ectodermal cells. 

 In Araphitrite, where, according to Mead, the division in the germ 

 bands is with spindles in the short axes, this method of division is per- 

 haps necessary to bring about the elongation of the germ bauds, and the 

 same is doubtless true in the germ bands of Crustacea. It is of course 

 not necessary', nor is it probable, that in these cases the position of the 

 spindle in the short axis is a reaction to the form of the cell ; more 

 probably the position of the spindle is detei'mined without reference to 

 this, as I have endeavored to show for the cells of Asplanchna. 



The conclusion to which I have coaie is therefore similar to that 

 maintained by Braem ('94), except that he seems to imply that the pur- 

 pose of the cleaving cells is always the same, viz. to gain free space for 

 the development of the pi'oducts of division, whereas it appears to me 

 that the facts indicate that the ends to be accomplished may be various, 

 and the means by which they are brought about equally varied. This 

 brings us to a nearer consideration of Braem's view. 



(3) Braem' s theory/ of least resistance. (Compare page 5.) — As just 

 stated, so for as Braem's view is teleological, I must agree with him ; but 

 in so far as he seems to restrict his teleology to the accomplishment of a 

 single purpose, — the attainment of the freest space for development, — 

 it seems to me that the facts are against him. Certainly the principle 

 of "least resistance" does not aid in understanding the cleavage of 

 Asplanchna, where a large number of the divisions take place in what 

 must be considered the direction of greatest pressure. Examination of 

 the figures will show that, as a rule, the blustomeres in the resting 

 stages are much flattened dorso-ventrally and extended laterally, as if 

 subjected to great pressure ; nevertheless, as shown in detail in the de- 

 scriptive portion and in the discussion of Hertwig's law, when division 

 takes place it is very frequently with the spindles in the dorso-ventral 

 axes. The cleavage in this direction seems to have a purpose, but that 

 purpose is not the gaining of the freest space for the development of the 

 products of division, but the accomplishment of the process of gastrulation. 

 Since Braem's principle is confessedly teleological, it was probably not 

 intended to be rigidly applicable to all cases; indeed, the author states 



