FERTILIZATION 179 



characters so easily observed in the living egg. It is also note- 

 worthy that frequently a radial or rotatory symmetry of 

 the egg is changed to a bilateral symmetry by the entrance 

 of the spermatozoon, and that usually the position of the 

 plane of bilateral symmetry is determined by the point at 

 which the sperm enters or by the path which the sperm takes 

 through the cytoplasm. And further this new plane of sym- 

 metry of the zygote coincides closely with the plane of the first 

 division of the zygote and with the median plane of the embryo 

 and adult. 



These aspects of organization and reorganization of the egg 

 are among the highly important aspects of development, and 

 largely determine many of the phenomena of subsequent 

 differentiation. They also illustrate the statement made in 

 the introductory chapter, that some of the most important 

 aspects of development are w^ra-cellular processes. We shall 

 return to this subject in a later chapter (Chapter VII). 



After the entrance of the spermatozoon and the consequent 

 redisposition of the substances of the cytoplasm, the course 

 of the immediately subsequent events is determined largely 

 by the state of the egg nucleus as regards its maturation process. 

 For in most cases maturation proceeds only to a certain point, 

 varying greatly in different forms, when it pauses, and is com- 

 pleted only after receiving the stimulus caused by the entering 

 sperm. Ordinarily the state of the egg is such that sperm can 

 gain admission only when this pause has been reached. 



In some cases, such as Nereis (Fig. 86), Ascaris (Fig. 94), 

 Cerebratulus (Fig. 95), and many Molluscs, it is true that the 

 sperm does enter the ovum before the maturation process has 

 even begun while the egg nucleus (germinal vesicle) is still 

 in the final resting stage preceding the first oocyte division. 

 In other cases most Molluscs, Thalassema, Sagitta, Teleosts, 

 the first polar spindle has formed and the first maturation 

 division may have reached the metaphase or anaphase, when 



relation \o the symmetry of the egg. a, anterior; c, clear protoplasm; cr, yellow 

 crescent; e, exoplasm or cortical layer, with yellow pigment; g.v, germinal vesicle 

 k, chorion; p, posterior; p. b, polar bodies; t, test cells; y, yolk (central gray mate- 

 rial); y.h, yellow hemisphere; d\ sperm pronucleus. 



