PROMO RPHOLOGICAL RELATIONS OF CLEAVAGE 287 



Stages. They are gradually established during the pre-embryonic 

 stages, and the (t^^, when ready for fertilization, has already accom- 

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



Mark, who was one of the first to examine this subject carefull)^ 

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

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

 phological features.^ The same view was very precisely formulated 

 by Watase in 1 891, in the following statement, which I believe to 

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

 at present that the ovum, which 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 pre-existing 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 pre-organized 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- 

 forms are concerned, though Watase does not enter into the question 

 as to how the arrangement of protoplasmic materials is effected. In 

 considering this question, we mu.st hold fast to the fundamental fact 

 that the ^g^ is a cell, like other cells, and that from an a priori point 

 of view there is every reason to believe that the cytoplasmic differ- 

 entiations that it undergoes must arise in essentially the same way as 

 in other cells. We know that such differentiations, whether in form 

 or in internal structure, show a definite relation to the environment 

 of the cell — to its fellows, to the source of food, and the like. We 

 know further, as Korschelt especially hds pointed out, that the egg- 

 axis, as expressed by the eccentricity of the germinal vesicle, often 

 shozvs a definite relation to the ovarian tissues, the germinal ^x^sicle 

 lying near the point of attachment or of food-supply. Mark made 



1 '81, p. 512. 2 '(ji^ p_ 280. 



