MUSEUM OF COMPARATIVE ZOOLOGY. 465 



visible in the interior of the vesicle. The latter at maturity approaches 

 the surface of the yolk and is dissolved before the egg leaves the ovarium. 

 In eggs treated with acetic acid the Richtungsspindel was observed to 

 have a peculiar structure. It was composed of a bundle of stout, short, 

 lustrous rods of -uniform thickness throughout, and so arranged as to 

 appear in optical cross-section as a circle of conspicuous granules. The 

 formation of two polar globules, and the subsequent appearance of an 

 " egg nucleus " '(at first as a small vacuole in the periphery of the yolk 

 under the polar globules), were observed in the living egg to follow each 

 other after intervals of a quarter of an hour only. In already excluded 

 eggs of Alciope a maturation spindle of considerable size was observed. 



The germinative dot in eggs of Ascidia intestinalis, as well as in Physo- 

 phora, in Sphserechinus, and in several mollusks (Unio, Tellina, Helix), 

 was found to be really composed of two substances, having, as in the 

 case of Asteracanthion, different physical and micro-chemical properties. 

 To designate these Hertwig uses the name Nudein^ for the larger, less 

 refringent, and usually enveloping substance ; and Paranuclein, for the 

 smaller body. As the names imply, he considers the former as the 

 essential part and the latter as the accessory part. This he does not- 

 withstanding the fact, already established by his studies on Asteracan- 

 thion, that the " Paranuclein " (as I conclude from the account of its 

 deportment in the two cases) is the part which is " in erster Linie " 

 engaged in the transmigratory changes accompanying the formation of 

 the first maturation spindle. Flemming, moreover, holds, as Hertwig 

 states, the reverse opinion as far as regards the case of lamellibranchs. 



Hertwig gives figures from his earlier studies on Hsemopis which now 

 have greater interest in view of his observations on the starfish. They 

 represent stages in the formation of the " Richtungsspindel " when por- 

 tions of the nuclear substance are still to be found in the vicinity of the 

 spindle figure. These bodies entirely disappear with the completion of 

 the spindle, i. e. by the time the polar globules begin to be formed. 



The formation of two polar globules in the sea-urchin (Sphserechinus 

 brevispinosus) takes place in nearly the same manner as in the star- 

 fish, except that the two maturation spindles and archiamphiasters are 

 larger. Hertwig acknowledges that his previous representations of the 

 metamorphosis of the germinative vesicle were produced from eggs in 

 a pathological condition. The reason why the maturation spindle was 

 not previously found in mature eggs is explained by the polar globules 

 being formed in the ovary, and at a time when the eggs do not pos- 

 sess a firm membrane, so that the latter are lost in the ovarial ifluid. 



VOL. VI. — NO. 12. 30 



