10 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
unimportant for the later morphogenetic processes. Gastrulation, for 
example, occurs in the same manner, after the most varied and 
fundamental alterations of the cleavage. Is this a fact which is 
capable of generalization, — of application to different animals and 
different methods of gastrulation? Doubtless the only positive answer 
to this question must come from experimental studies; but a care- 
ful descriptive analysis of the process in Asplanchna gives results 
which, if the egg were a mechanism of the ordinary physical sort, would 
be definite and conclusive. 
II. Descriptive Portion. 
1. Form AND STRUCTURE OF THE ÉGG. 
The development of the embryo in Asplanchna Herrickii takes place 
within the body of the mother, the egg lying enclosed in the enlarged 
oviduct, close to the ovary. The chief axis of the developing embryo 
bears no relation to the position of surrounding organs of the mother, 
the egg lying in the oviduct as it might within a protecting sac of 
any foreign material, its position determined by chance circumstances. 
In cases where two embryos are present, their axes may make any angle 
with each other. 
For study it is necessary to dissect out the eggs. <A full account of 
the methods of work is given in Part Third; here it is important to note 
two facts: (1) All the work was done on preserved material; (2) Each 
egg comes from a different individual, and is therefore in at least a 
slightly different stage from every other. A considerable number of 
eggs showing any given process, as, for example, the first cleavage, gives 
therefore a series of stages, so that a complete idea of the changes taking 
place may be gained. 
The unsegmented egg is approximately an ellipsoid of nearly equal 
axes, one end often slightly more pointed than the other. The form and 
proportions vary a little, as do also the absolute dimensions. In many 
eggs it is difficult to distinguish a more pointed end. The proportion of 
the longer to the shorter axis is about as 9 to 8, and the average dimen- 
sions of the egg are about 90 u through the longer axis by 80 u through 
the shorter. Variations from a minimum of 84 by 70 to a maximum 
of 97 u by 83 u were observed. 
Whether an egg membrane is present or not is exceedingly difficult to 
decide ; and I have not succeeded in thoroughly satisfying myself upon 
