92 GENERAL EMBRYOLOGY 



as it was placed in the epithelium of the gonad, the chief 

 egg axis corresponding with the axis passing through the 

 attached and free surfaces of the epithelial cell. This polarity 

 apparently determines the primary position of the egg nucleus 

 and centrosomes, and thus secondarily determines also the 

 arrangement of the cytoplasmic substances which develop 

 through the interactions between the nucleus and cytoplasm, 

 processes which may frequently be observed during the growth 

 period of the ovum. The two poles of the egg are commonly 

 unlike (Figs. 42, 48), so that we distinguish an animal and a 

 vegetal pole, corresponding in most cases with the originally 

 free and attached surfaces respectively, although this relation 

 may occasionally be reversed (Echinoderms). In general 

 the animal pole is that toward which the nucleus is eccentrically 

 displaced, and nearer which the centrosome, or similar body, is 

 located; it is the more protoplasmic, and therefore the more 

 active region of the egg. The vegetal pole is frequently occu- 

 pied largely by the relatively inert food substance, the material^ 

 in general related with the vegetative organs of the developing 

 embryo. 



As regards the nature of the organization, or promorphological 

 relations, of the egg, two views have been taken and will be discussed 

 in Chapter VII. The first is that the differentiated substances visible 

 within the cytoplasm are genuinely " organ-forming substances," or at 

 any rate tissue forming, in their potentiality. Thus they represent the 

 organs of the embryo in an intracellular form. The second view is that 

 these substances are only secondarily related to the real morphology 

 of the embryo, and that both embryonic structure and the differentiated 

 substances of the egg, are the result of an underlying, invisible, and as 

 yet little-known organization of the ground substance of the egg cyto- 

 plasm. According to this view the correspondence between the ' 'organ- 

 forming substances" of the ovum, and the organs and tissues of the 

 embryo, is not in itself direct, but results from their common relation to 

 the primary underlying arrangement or organization of the substance 

 of the egg. In some cases the arrangement and position of these sub- 

 stances may be considerably altered experimentally without disturbing 

 the normal course of development. It should be added, however, that 

 in some other cases such a disarrangement does effect a corresponding 

 disarrangement of the organs or tissues of the embryo. 



