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JENNINGS: DEVELOPMENT OF ASPLANCHNA HERRIOCKII. 69 
But mere inerease 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 Amphitrite, 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 bands, 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 determined without reference to 
this, as I have endeavored to show for the cells of Asplanchna. 
The conclusion to which I have come 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 products 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 far as Braem’s view is teleological, I must agree with him; but 
in so far as he seems to restriet 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 blastomeres 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- 
seriptive 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 tho freest space for the development of the 
produets 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 
