288 CELL-DIVISION AND DEVELOPMENT 



the pregnant suggestion, in 1881, that the primary polarity of the Qg^ 

 might be determined by " the topographical relation of the egg (when 

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

 sue from which it is differentiated;' and 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 &gg 

 is directly developed from epithehal cells, this relationship were 

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

 ing thouo-h 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 p. 40, the nucleus usually lies towards the base of the 

 cell, near the source of food, while the characteristic cytoplasmic 

 products, such as zymogen granules and other secretions, appear in 

 the outer portion.'^ The exact conditions under which the ovarian 

 (tgg develops are still too little known to allow of a positive conclu- 

 sion regarding Mark's suggestion. Moreover, 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 towards 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 

 to the periphery, the deutoplasm become massed at one pole, and 

 the polarity of the egg come into view {Nereis, Figs. 43 and 71).^ In 

 such cases the axis of the egg is not improbably predetermined by 

 the position of the centrosome, and we have still to seek the causes 

 by which the position is established in the ovarian history of the Qgg. 

 These considerations show that the problem is a complex one, involv- 

 ing, as it does, the whole question of cell-polarity ; and I know of 

 no more promising field of investigation than the ovarian history of 

 the ovum with reference to this question. That Mark's view is cor- 

 rect in principle is indicated by a great array of general evidence 

 considered in the following chapter, where its bearing on the general 

 theory of development is more fully dealt with. 



1 '81, p. 515. ^ Hatschek has suggested the same comparison {Zoologie, p. 112). 



'^ The immature egg of Nereis shows, however, a distinct polarity in the arrangement of 

 the fat-drops, which form a ring in the equatorial region. 



