JENNINGS: DEVELOPMENT OF ASPLANCHNA HERRICKII. 109 



of the cytoplasm'? (c) "What determines the rotation of the asters and 

 nucleus as the cell passes into the karyokiuetic condition ] 



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 reaction 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 Asplauchna 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 cleav- 

 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 (?). 



(e) The changes in form taking place as the cells divide. 



1 It 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. 

 Tiie origin of the process of gastrulation in phylogeny is not touched by this 

 account. It is a common phenomenon in the organic 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 field of the present 

 paper. 



