10 BULLETIN: MUSEUM OF COMPARATR^E 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 

 ditfereut methods of gastrulation 1 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 detiuite and conclusive. 



II. Descriptive Portion. 



1. Form and Structure of the Egg. 



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 unsegniented 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 /x through the longer axis by 80 fj. through 

 the shorter. Variations from a minimum of 84 /a by 70 /a to a maximum 

 of 97 /A by 83 /x 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 



