MUSEUM OF COMPARATIVE ZOOLOGY. 449 



first "embryonic sphere," — the analogue of the germinative dot. Its 

 contour at length becomes less distinct and it entirely disappears. 

 With the dissolution of the nucleus the existence of the first embryonic 

 sphere is at an end, although some minutes later there begin to be 

 developed in the yolk certain phenomena which lead to the formation 

 of two new nuclei and to the division of the yolk into two embryonic 

 spheres. The yolk is now homogeneous. Little by little a protoplasmic 

 mass is collected in the centre ; this increases and becomes more readily 

 visible, but its contours are blended ; the granules cease their rotation 

 and are disposed in rays. The central body becomes elongated in a 

 plane perpendicular to that of the polar globules ; the motion is not 

 amoeboid ; this nuclear body is divided by a constriction, and afterwards 

 the yolk suffers the same fate. 



BiJTSCHLi (77^ pp. 232 - 237, Taf. XVII.), independently of the recent 

 observations of Hertwig and Fol, radically modified his opinion of the 

 nature of the polar globules. In Neritina fluviatilis he finds that both 

 the fertile and the mfertile eggs of a capsule produce polar globules ; the 

 former at least three (only one observation), the latter a larger number, 

 sometimes as many as five. It is not possible, he says, to be certain that 

 all the globules are observed, since in opening the capsule they may 

 easily be lost. Staining in Beale^s carmine and the subsequent well- 

 known method of decoloration by means of hydrochloric acid furnishes 

 evidence that the polar globules are not composed exclusively of nuclear 

 substance, but that they are each composed of protoplasm which encloses 

 from one to three small nuclei ; and, further, that the infertile yolk after 

 the formation of the polar globules still embraces from one to three 

 small nuclei, — in other words, that a part at least of the germinative 

 vesicle remains in the yolk after the production of the polar glohides. 

 Biitschli fully accepts 0. Hertwig's view of the origin and nature of 

 the polar globules, but still from a physiological standpoint thinks their 

 principal signification is to be sought in the removal of a portion of 

 the Qg^ nucleus (germinative vesicle), whether this is accomplished 

 directly or under the form of a " Zellknospung." 



It is probable, he adds, that the infertile eggs have remained unfe- 

 cundated. If this be true, Neritina will afford evidence that polar 

 globules may be produced by z«7ifecundated eggs, a conclusion which 

 Fol and Hertwig have likewise reached from satisfactory evidence. In 

 view of their extensive prevalence, the polar globules are probably of 

 fundamental significance ; their import will receive a sufficient explana- 

 tion only with a more intimate knowledge of the processes of reproduc- 



voL. VI. — NO. 12. 29 



