PROMORFIIOLOGICAI. RELATIONS OF CLEAVAGE 387 



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 must hold fast to the fundamental fact 

 that the ^gg 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 has pointed out, that the egg- 

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

 sJiows a definite rrlation to the ovarian tissues, the germinal vesicle 

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

 the pregnant suggestion, in 1881, that the primary polarity of the egg 

 might be determined by "-the topographical relation of the ^'^^ (when 

 still in an indifferent state) to the remaining cells of the maternal tis- 

 sue from ivhich it is differentiated,'" ■^nd added that this relation might 

 operate through the nutrition of the ovum. " It would certainly be 

 interesting to know if that phase of polar differentiation which is 

 manifest in the position of the nutritive substance and of the germi- 

 nal vesicle bears a constant relation to the free surface of the epithe- 

 lium from which the egg takes its origin. If, in cases where the egg 

 is directly developed from epithelial cells, this relationship were 

 demonstrable, it would be fair to infer the existence of correspond- 

 ing, though obscured, relations in those cases where (as, for example, 

 in mammals) the origin of the ovum is less directly traceable to an 

 epithelial surface."^ The polarity of the- egg would therefore be 

 comparable to the polarity of epithelial or gland-cells, where, as 

 pointed out at page 57, the nucleus usually lies toward the base of the 

 cell, near the source of food, while the centrosomes, and often also 

 characteristic cytoplasmic products, such as zymogen granules and 

 other secretions, appear in the outer portion. ^ The exact conditions 

 under which the ovarian egg develops are still too little known to 

 allow of a positive conclusion regarding Mark's suggestion. More- 

 over, the force of Korschelt's observation is weakened by the fact that 

 in many eggs of the extreme telolecithal type, where the polarity is 

 very marked, the germinal vesicle occupies a central or sub-central 

 position during the period of yolk-formation and only moves toward 

 the periphery near the time of maturation. 



Indeed, in mollusks, annelids, and many other cases, the germinal 

 vesicle remains in a central position, surrounded by yolk on all sides, 

 until the spermatozoon enters. Only then does the egg-nucleus move 



1 '81, p. 515. 



'^ Hatschek has suggested the same comparison (Zootoo/e, p. 112). 



