JENNINGS: DEVELOPMENT OF ASPLANCHNA HERRICKIL 109 
of the cytoplasm? (e) What determines the rotation of the asters and 
nucleus as the cell passes into the karyokinetic condition ? 
EH 
25. It may be concluded from 20, 21, 22, and 24 that the final posi- 
tion of the spindle and manner of cleavage are causally determined 
by processes — of an unknown character taking place within the 
protoplasm. 
26. The definite relation of the position of the spindle to external 
conditions observed in some cases — such as to the form of the cell, the 
direction of pressure (), and the direction of the incoming rays of light — 
is to be interpreted as a reaetion to stimulus, dependent in every case 
upon the specific structure of the protoplasm, and variable with that 
Structure. 
27. The manner of division is related to the purpose to be attained 
by the given division, and to the general morphogenetic changes in the 
organism. In Asplanchna the method of cleavage is adapted to bring- 
ing about gastrulation, 
28. It follows from 16 that cleavage is not merely a quantitative 
division into similar units; it is accompanied by other developmental 
processes, some of which are distinctly traceable. 
29. Gastrulation in Asplanchna is not a process distinct from cleay- 
age, but is an accompaniment and a result of cleavage. The process 
of which it forms a part begins at the third cleavage and is not finished 
until much later than what is commonly spoken of as gastrulation 
proper. 
30. Gastrulation in Asplanchna may be analyzed into several factors. 
(a) The form of the egg, or the influences determining it. 
(b) The direction of cleavage. 
(c) The inequality of cleavage. 
(d) The sequence of cleavage (öh. 
(e) The changes in form taking place as the cells divide. 
1 Tt may be well to state expressly that I do not consider the above as in any 
sense a general explanation of the process of gastrulation. My aim has been to 
give as nearly as possible a correct account, from the standpoint of developmental 
mechanics alone, of. the facts in regard to the early development of a single form. 
The origin of the process of gastrulation in phylogeny is not touched by this 
account. It is a common.phenomenon in the organie world, that the same end is 
accomplished by different means in different cases; doubtless in many forms gas- 
trulation is brought about in a way that bears no resemblance to the process in 
Asplanchna. In general, the whole question of the origin of processes to which 
an end or purpose can be assigned lies entirely without the fleld of the present 
paper. 
