386 CELL-DIVISION AND DEVELOPMENT 



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

 ment of the ^^g may give rise to a complete larva (p. 353). 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 Pfluger ('84), 

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

 may be entirely rearranged under the influence of gravity, and a 

 new axis established. In sea-urchins, my own observations ('95) 

 render it probable that the egg-axis is not finally established until 

 after fertilization. These and other facts, to be more fully considered 

 in the following chapter, give strong ground for the conclusion that 

 the promorphological features of the ^gg are as truly a result of 

 development as the characters coming into view at later stages. They 

 are gradually established during the preembryonic stages, and the 

 ^gg, when ready for fertilization, has already accomplished part of 

 its task by laying the basis for what is to come. 



Mark, who was one of the first to examine this subject carefully, 

 concluded that the ovum is at first an indifferent or homaxial cell 

 {i.e. isotropic), which afterward acquires polarity and other promor- 

 phological features.^ The same view was very precisely formulated 

 by Watase in 1891, in the following statement, which I believe to 

 express accurately the truth : " It appears to me admissible to say 

 at present that the ovum, w^hich may start out without any definite 

 axis at first, may acquire it later, and at the moment ready for its 

 cleavage the distribution of its protoplasmic substances may be such 

 as to exhibit a perfect symmetry, and the furrows of cleavage may 

 have a certain definite relation to the inherent arrangement of the 

 protoplasmic substances which constitute the ovum. Hence, in a 

 certain case, the plane of the first cleavage-furrow may coincide with 

 the plane of the median axis of the embryo, and the sundering of 

 the protoplasmic material may take place into right and left, accord- 

 ing to the preexisting organization of the ^gg at the time of cleav- 

 age ; and in another case the first cleavage may roughly correspond 

 to the differentiation of the ectoderm and the entoderm, also accord- 

 ing to the preorganized constitution of the protoplasmic materials of 

 the ovum. 



" It does not appear strange, therefore, that we may detect a cer- 

 tain structural differentiation in the unsegmented ovum, with all the 

 axes foreshadowed in it, and the axial symmetry of the embryonic 

 organism identical with that of the adult." ^ 



This passage contains, I believe, the gist of the whole matter, as 

 far as the promorphological relations of the ovum and of cleavage- 



1 '81, p. 512. 2 'gi^ p_ 2S0. 



