MUSEUM OF COMPARATIVE ZOOLOGY. 481 



cles of considerable size. By the migration of the peripheral vesicle 

 they come in contact, and become flattened against each other. After 

 treatment with acetic acid each appears to consist of a compact cortical 

 layer and of fluid contents which are traversed by netlike cords with 

 nodular swellings, and in which are found clusters of granules. This is 

 probably an artificial production, since in preparations made with osmic 

 acid the nuclear contents remain homogeneous, and are only limited by 

 a somewhat firmer cortical layer. Since these nuclei were not seen to 

 become confluent, — as is elsewhere (p. 328) more fully described, — 

 until evidences of the first segmentation appeared, Hertwig concludes 

 that the period of their confluence is of limited duration. 



The following important conclusions are reached. The peripheral 

 nucleus arises from the granules of the lateral zone of thickenings, as in 

 ordinary cell-division, inasmuch as these granules are by the reception 

 of nuclear fluid converted into vacuoles, which ultimately become fused. 

 Therefore these vacuoles are not so many isolated nuclei (as Biitschli 

 thinks), but the component elements of a single nuclear structure. 

 But, as the spindle was derived from the nuclear substance of the ger- 

 minative vesicle, it follows that there exists an uninterrupted connection 

 between the several generations of nuclei from the germinative vesicle to the 

 nucleus of segmentation. The direct evidence of the origin of the isolated 

 stellate figure is wanting, but from analogy with Toxopneustes there is 

 reason to believe that it is produced by the nucleus of a spermatozoon 

 which has penetrated the yolk. Therefore the segmentation nucleus is 

 derived from the conjugation of two sexually different nuclei ; a female 

 nucleus, descended from the germinative vesicle, and a male nucleus, derived 

 from the body of a spermatozoon. 



Finally, the formation of the polar globules takes place before fecun- 

 dation, since the latter is really accomplished only when the confluence 

 of the male and female nuclei takes place. This coincides, moreover, 

 with Strasburger's studies on " canal cells." Whether the pinching off 

 of these globules may not be affected by the act of fecundation, cannot 

 be so positively answered. 



After artificial impregnation the eggs of the frog all exhibit, according 

 to Hertwig, a change at the pigmented pole, which is readily distinguish- 

 able with a hand-lens. The middle of the dark field appears clearer and 

 yellowish, as though veiled in a layer of unpigmented substance. This 

 is really a thin layer of finely granular substance (with uneven surface 

 and thickest at its middle point) which closely resembles the. contents of 

 the germinative vesicle in its last observed stages. There are in it also 



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