MUSEUM OF COMPARATIVE ZOOLOGY. 509 



ence of a potentially or prospectively nuclear substance outside the 

 corpuscles in question, does not involve the necessity of considering 

 such unsegregated substance to be a veritable nucleus. 



Hovi'ever unsatisfactory this attempt to harmonize these two cases 

 may appear, I cannot hesitate to adopt it, if — as seems to be the case 

 at present — the only other alternative is to hold the bodies under con- 

 sideration to be in the case of Limax proiiucleoii. 



0. Hertwig ('78") has established in Mitrocoma the existence of a 

 vacuole much smaller than that which is derived from the inner half of 

 the second maturation spindle, and considers it to be the sperm nucleus, 

 even though no radial structure was discovered in the surrounding pro- 

 toplasm. He thinks this may be due to the difficulty of recognizing 

 such a radial structure when the protoplasm is homogeneous, as it is 

 here. From the evidence thus far gained in Limax it would seem that in 

 other cases it might not be possible to distinguish the stellate structure, 

 and that, too, where Hertwig's explanation would be insufficient. These 

 two nuclear vacuoles, he continues, become mutually flattened, and in 

 the living Qg^ suddenly cease to be visible, but in their place may be 

 found, after treatment with acetic acid, the customary spindle and 

 amphiaster. 



In artificially fertilized eggs of Sagitta the yolk, before the first polar 

 globule is formed, withdraws somewhat from the vitelline membrane, 

 and a faint radiation may be detected at the vegetative pole. This 

 remains, as in the starfish, inconspicuous until the second polar globule 

 begins to be formed, when it increases in extent, and migrates toward 

 the centre of the yolk. It appears, says Hertwig, as though the plasma 

 when ruled by the process of division going on at the animal pole, could 

 not respond to the stimulus of the male nucleus in so liberal a manner 

 as later. 



Interesting features of the conjugation of the two pronuclei in Sagitta 

 are, that as they approach each other they become pointed, and that 

 the pointed ends become applied to each other and assume a darker as- 

 pect (see Hertwig's Taf. X. Fig. 18), as though the more compact ele- 

 ments were collected there, and the more fluid had receded to the 

 opposite ends of the pronuclei. As far as relates to the altered form 



plasm, in which the amount of infused nuclear substance at any given point is pro- 

 portional to its nearness to the central corpuscle. The gradual transition in the 

 nature of this ** nucleoplasm " till it is no longer distinguishable from the surround- 

 ing yolk protoplasm, appears to me an important objection to considering it the 

 nucleus. 



