JENNINGS: DEVELOPMENT OF ASPLANCHNA HERRICKII. 3 
Furthermore, there is much discussion of the question as to whether 
cleavage 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 morphogenetie 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 examplo, 
Ts 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 formative 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 Rotifera 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 may be 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 Rotifera, 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, 80 far as traced, with the develop- 
ment of other Rotifera, 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. 
