No. I.] THE EMBRYOLOGY OF THE UNIONIDAE. 43 



That is, in which the entomeres lie anterior, posterior, right 

 and left respectively. If the oblique nature of the cleavage 

 remains the same in these forms as in those above cited, the 

 relation of the various generations of micromeres also to the 

 embryonic axes must be different. The best studied forms 

 which show these relations are Clepsine, Planorbis, and Neritina. 

 In Clepsine (Whitman, Nos. 61 and 62), for instance d^ (x' of 

 Whitman) the neuro-nephroblast, or first somatoblast, does not 

 lie in the middle line at first, but to the right (Whitman, Figs. 

 26, 33, and 34) ; for some time its products are asymmetrically 

 arranged, but gradually become shifted into bilateral symmetry. 

 In Clepsine Z> forms the mesoblast immediately after the 

 budding off of ^^ There may be some correlation between 

 these anomalies of cytogeny and the reversed relations of the 

 blastomere generations to the embryonic axes. But at present 

 we are unable to explain why, when widely separated forms 

 agree, nearly related species should show reversed relations. 

 The first careful study of the second condition will no doubt 

 throw much light on this subject. 



Studies of cell-lineage have an important bearing on the 

 nature of differentiation. Any one who watches a segmenting 

 ovum sees the differentiation of the parts which ultimately 

 compose the adult organism going on under his eyes. It is too 

 soon to say that mere observation of the phenomena accom- 

 panying differentiation will teach us nothing of its determining 

 factors. As well say that the mere study of the facts of varia- 

 tion will teach us nothing as to the causes of variation! It 

 is a hopeful sign that baseless hypotheses as to the nature of 

 both these phenomena are giving way to a patient study of the 

 accompanying facts. 



A tremendous advance has been made since the time when 

 it was thought sufficient to say, "by a series of rapidly succeed- 

 ing cell-divisions the ovum is cut up into a great number of 

 segmentation spheres, which arrange themselves in the form of 

 a hollow ball." The first segmentation plane has since been 

 shown often to have a definite relation to the future axes of the 

 body ; the various cells are not undifferentiated or equivalent, 

 but destined for definite positions and functions in the larval 



