rKOMORPHOLOGR'Al. DELATIONS OF CLEAVAGE 285 



2. Meaning of the Promorphology of tJic Ovum 



The interpretation of the promorphology of the ovum cannot be 

 adequately treated apart from the general discussion of development 

 given in the following chapter ; nevertheless it may conveniently be 

 considered at this point. Two fundamentally different interpreta- 

 tions of the facts have been given. On the one hand, it has been 

 suggested by Flemming and Van Beneden, 1 and urged especially 

 by Whitman, 2 that the cytoplasm of the ovum possesses a definite 

 primordial organization which exists from the beginning of its exis- 

 tence even though invisible, and is revealed to observation through 

 polar differentiation, bilateral symmetry, and other obvious characters 

 in the unsegmented egg. On the other hand, it has been maintained 

 by Pfliiger, Mark, Oscar Hertwig, Driesch, Watase, and the writer 

 that all the promorphological features of the ovum are of secondary 

 origin ; that the egg-cytoplasm is at the beginning isotropous i.e. 

 indifferent or homaxial and gradually acquires its promorphological 

 features during its pre-embryonic history. Thus the egg of a bilateral 

 animal is not at the beginning actually, but only potentially, bilateral. 

 Bilaterality once established, however, it forms as it were the mould 

 in which the cleavage and other operations of development are cast. 



I believe that the evidence at our command weighs heavily on 

 the side of the second view, and that the first hypothesis fails to 

 take sufficient account of the fact that development does not nec- 

 essarily begin with fertilization or cleavage, but may begin at a far 

 earlier period during ovarian life. As far as the visible promorpho- 

 logical features of the ovum are concerned, this conclusion is beyond 

 question. The only question that has any meaning is whether these 

 visible characters are merely the expression of a more subtle pre- 

 existing invisible organization of the same kind. I do not believe 

 that this question can be answered in the affirmative save by the 

 trite and, from this point of view, barren statement that every effect 

 must have its pre-existing cause. That the egg possesses no fixed 

 and predetermined cytoplasmic localization with reference to the 

 adult parts, has, I think, been demonstrated through the remarkable 

 experiments of Driesch, Roux, and Boveri, which show that a frag- 

 ment of the egg may give rise to a complete larva (p. 308). There 

 is strong evidence, moreover, that the egg-axis is not primordial, but 

 is established at a particular period; and even after its establishment 

 it may be entirely altered by new conditions. This is proved, for 

 example, by the case of the frog's egg, in which, as Pfliiger ('84), 

 Born ('85), and Schultze ('94) have* shown, the cytoplasmic materials 



1 See p. 298. 2 Cf. pp. 299, 300. 



