472 BULLETIN OF THE 



structure, justify the opinion that the single nucleus found in the egg 

 immediately before segmentation, and surrounded hy rays of yolk granules, 

 is the result of the copidation of two nuclei. Hertwig also reports that, 

 while in most cases only one clear spot makes its appearance in the pe- 

 riphery of the yolk, occasionally more (up to four) have been observed to 

 make their way to the Qgg nucleus ; but after the appearance of anoma- 

 lous nuclear figures, the eggs soon perished. It is therefore probable 

 that these eggs were from the beginning pathologically altered. 



In the interpretation of these observations he concludes that the con- 

 stancy of their appearance at a uniform interval after the mingling of the 

 sexual elements is evidence that they are dependent on fertilization. 

 From this and the observed filament it is not to be doubted that these 

 changes are referable to the penetration into the yolk of a sperma- 

 tozoon, of which the tail is the observed filament, while the head (its 

 nucleus) becomes the " Spermakern." The tail is probably dissolved 

 either at once or during the migration of the sperm nucleus. The 

 homogeneous protoplasmic area and the radial figure are apparently 

 induced by the sperm nucleus which occupies their centre, in the fol- 

 lowing way : the nucleus exerts an attractive influence on the homoge- 

 neous components of the yolk, which thus become most densely collected 

 around the nucleus, and thence radiate in all directions. The yolk gran- 

 ules passively assume a position in the interstices between the rays of 

 the attracted substance. 



The most important part of fecundation, hitherto explained as the 

 copulation of two cells,* is found in the fusion of the two nuclei from 

 which "arises first a nucleus (nucleus of the first cleavage-sphere) 

 equipped with living forces, which eff'ectively stimulates, and in many 

 respects controls, the further process of development in the yolk." 



In a foot-note (p. 386) Hertwig calls attention to the fact that for the 

 time being the egg cell may be considered as in an hermaphroditic condi- 

 tion, inasmucli as two sexually different nuclei are present in a common 

 protoplasmic mass. Further, since the "nucleus" and the "nucleolus" 

 of Infusoria are, from the changes they undergo in reproduction, com- 

 parable with the egg nucleus and sperm nucleus respectively, it follows 

 that the Infusoria may be considered as hermaphroditic unicellular organ- 

 isms, inasmuch as the sexual diff^erentiation of the nuclear substance, 

 which has been accomplished in other organisms in two separate cells, is 

 with them effected in a single cell. 



In a foot-note Bijtschli ('75^ p. 109) says his recent studies tend to 



* See Haeckel '74, pp. 135 - 138, and '75, pp. 482, 483. 



