JENNINGS: DEVELOPMENT OF ASPLANCHNA HERPJGKII. 3 



Furthermore, there is much discussion of the question as to whether 

 cleava^'e is a mere quantitative separation of a single mass into smaller 

 masses similar in nature to each other and to the original egg, or 

 whether it is accompanied by a difFerentiation of the separated blasto- 

 meres, — as a result either of qualitative division or other changes. 



A third question of theoretical interest, somewhat related to the last, 

 is whether the method of cleavage has a direct mechanical relation to 

 future morphogenetic processes, or whether it is merely the passing of 

 partitions through a mass of protoplasm, the order in which this occurs 

 and the arrangement of the partitions being immaterial. For example. 

 Is gastrulation a process independent of cleavage and merely requiring 

 the latter as a prerequisite, — as the planting of seeds must be preceded 

 by ploughing, — or is gastrulation in some way connected with or depend- 

 ent upon the manner of cleavage ? Stated in the most general terms, 

 this is the question : Is cell division a direct morphogenetic factor, or 

 are the real foi*mative processes dependent upon the introduction of 

 other factors after the cleavage is finished % 



With these questions in mind, I have studied the development of an 

 organism of the class Eotifera throughout those stages of development 

 in which it is possible to make the cells the units of observation, — that 

 is, through cleavage and gastrulation and somewhat later. 



Broadly stated, the object of the work maybe expressed as the analysis 

 of the early development of an organism into the simplest factors possible. 



The development of Asplanchna Herrickii has not been studied previ- 

 ously, and in the course of this paper it will be necessary to discuss some 

 matters which are of importance primarily to persons who are engaged 

 particularly with the morphology of the Eotifera, and which are not of 

 especial interest from a morphogenetic standpoint. In order to dis- 

 tinguish these two lines of discussion, I shall divide the work into two 

 main portions. Part First will contain all matters bearing upon devel- 

 opmental mechanics. Here will be found the minute description of the 

 cleavage, gastrulation, and other processes, as well as a discussion of 

 their bearing upon the problems of morphogenesis. Part Second will 

 contain a brief review of previous knowledge of the organism studied, 

 a comparison of the development, so far as traced, with the developn 

 ment of other Ptotifera, and a discussion of some of the conditions de- 

 scribed by other authors. These principal parts will be followed by a 

 third, on material, methods, and other subordinate matters, and the 

 ■whole will be closed by a summary of the more important conclusions 

 arrived at. 



