No. 2.] ORGANIZATION OF THE EGG OF UNIO. 263 



cleavage in the cells of the &%g of Unio, and of other animals 

 possessing strictly determinate cleavage, is a prospective one. 

 " In the cleavage of the Q.^g of Unio there are marked varia- 

 tions in the size of the cells and in the rate and direction of 

 their cleavages ; in every case these possess prospective sig- 

 nificance, and by means of them the organism is able, so to 

 speak, to realize, in the most direct manner possible, on its 

 available capital, the substance of the &g^. To this principle I 

 have given the name of adaptation in cleavage'' {I.e., p. 53). 

 I must refer to this paper and to an earlier one, " Embryology 

 of the Unionidae'' ('95), for the details of proof that there is no 

 relation between the form of the cleavage and the distribution 

 of the yolk or any extrinsic forces. 



The larger of the first two cells is destined to form the pos- 

 terior part of the embryo, including the mesoblast and the 

 immense shell gland. The smaller cell will form the relatively 

 small anterior portion of the embryo. The proportions of 

 these parts are, then, marked out by the first cleavage. The 

 place of the first cleavage is due to the position ultimately 

 occupied by the first cleavage spindle. The manner in which 

 this reaches its excentric position has already been described 

 in detail. It is certainly due to inherent forces of the undi- 

 vided protoplasm. Before the Q.gg of Unio has divided for 

 the first time it thus foreshadows not only the polarity (posi- 

 tion of ectoderm and endoderm), but also the position and 

 proportio7is of anterior and posterior parts of the embryo. 

 TJiat is to say, the forces witJmi the tmseg^nented egg of Unio 

 are those 7iot only of a bilaterally symmetrical ani^nal, but of 

 such an animal of definite proportions. This much at least is 

 evident ; how far other features of the embryo are actually 

 existent in the unsegmented ^g^, and how far they arise in 

 response to subsequent conditions, internal and external, I am 

 unable to determine. I simply repeat that, independently of 

 cleavage or other of the phenomena usually styled develop- 

 mental, the bilaterality and the proportions of the embryo of 

 Unio are dependent on a certain distribution of protoplasmic 

 (as opposed to nuclear) forces. Nor are these forces in any 

 way complicated or modified by subsequent events ; rather is 



