MATURATION OF GAMETES 161 



future individual arises (Fankhauser, '48). Within the cytoplasm of the ma- 

 ture egg of many chordates, this inherent organization is revealed at the 

 time of fertilization by the appearance of definite areas of presumptive 

 organ-forming substances. For example, in the egg of the frog and other 

 amphibia, the yolk pole is the stuff from which the future entodermal struc- 

 tures take their origin; the darkly pigmented animal or nuclear pole will 

 eventually give origin to epidermal and neural tissues; and from the zone 

 between these two areas mesodermal and notochordal tissues will arise (fig. 

 119K). Similar major organ-forming areas in the recently fertilized egg have 

 been demonstrated in other chordates, as in the ascidian, Styela, and in the 

 cephalochordate, Amphioxus. In the eggs of reptiles, birds, and teleost and 

 elasmobranch fishes, while the relationship to the yolk is somewhat different, 

 major organ-forming areas of a similar character have been demonstrated at 

 a later period of development (Chaps. 6-9). This suggests that these eggs 

 also possess a fundamental organization similar, although not identical, to 

 that in the amphibian egg. 



4) Polarity of the Egg and Its Relation to Body Organization and Bilateral 

 Symmetry of the Mature Egg. One of the characteristic features of the ter- 

 minal phase of egg differentiation in the chordate group is the migration of 

 the germinal vesicle toward the animal pole of the egg (figs. 72F, 11 9A). 

 As stated above, in many vertebrate eggs the deutoplasmic material becomes 

 situated at the opposite pole, known as the vegetal (vegetative) or yolk pole, 

 either before fertilization or shortly after. The relatively yolk-free protoplasm 

 aggregates at the animal pole. Consequently the maturation divisions of the 

 egg occur at this pole (fig. 1 19 A, B, D). The formation of a definite polarity 

 of the egg, therefore, is one of the main results of the differentiation of the 

 oocyte. 



Various theories have been suggested in an endeavor to explain polarity 

 in the fully developed egg or oocyte. All these theories emphasize qualitative 

 and quantitative differences in the cytoplasmic substances extending from 

 one pole of the egg to the other (fig. 90). 



The animal and vegetal poles of the egg have a definite relationship to 

 the organization of the chordate embryo. In Amphioxus, the animal pole 

 becomes the ventro-anterior part of the embryo, while in the frog the animal 

 pole area becomes the cephalic end of the future tadpole, and the yolk pole 

 comes to occupy the posterior aspect. In teleost and elasmobranch fishes 

 the yolk-laden pole lies in the future ventral aspect of the embryo, and it 

 occupies a similar position in the reptile, bird, and prototherian mammal 

 (see fig. 215). Studies have shown that the early auxiliary or trophoblastic 

 cells in eutherian mammals lie on the ventral aspect of the future embryo. 

 Consequently, it is to be observed that the various substances in mature 

 vertebrate and protochordate eggs tend to assume a polarized relationship 

 to the future embryonic axis and body organization. 



