MUSEUM OF COMPARATIVE ZOOLOGY. 291 



From Fol's ('75", pp. 108-112) memoir on the Pteropods one learns 

 that the nucleus of the fecundated vitellus, or the germinative vesicle (as 

 he still continues to call the primary cleavage nucleus), arises by the 

 fusion of two or three corpuscles at the centre of a molecular aster whose 

 rays disappear during the fusion. This nucleus, which attains some- 

 times a third the diameter of the whole vitellus, occupies the centre of 

 the formative protoplasm ; the latter is smaller than the nutritive por- 

 tion of the vitellus, and the two meet in a plane. Although it shows a 

 fine stippling, the nucleus is much more homogeneous than the proto- 

 plasm itself, and also less fluid. The nucleolus is always wanting. A 

 nuclear membrane can be demonstrated easily after contact with sea- 

 water or reagents, but this does not warrant the conclusion that it exists 

 in the living condition. 



After an interval of repose the nucleus disappears ; but just before it 

 ceases to be visible there appear on opposite sides, at the boundary be- 

 tween it and the protoplasm, two points, which differ but little from it 

 in refractive power. Straight rays soon diverge from these points into its 

 interior : they rapidly increase in number, and become elongated until 

 those from the opposite sides meet in the middle of the nucleus, which 

 at this moment disappears. No trace of rays is to be seen outside the 

 nuclear vesicle in the living egg, but acetic acid causes also this portion 

 of the aster to appear. If applied before the nucleus becomes invisible, 

 the acid causes it, contrary to the case of Geryonia, to disappear."* The 

 rays extend to near the periphery. The central part of each star is 

 easily distinguishable without the use of acid, but is more distinct with 

 it. The rays occupying the place formerly filled by the nucleus are often 

 inflected, and pass from one star to the other. Soon after the nucleus 

 disappears, the stars move apart. After a time a furrow is to be seen on 

 the surface of the yolk running at right angles to a line joining them. 

 Acetic acid develops, just before the appearance of the furrow, a very 

 distinct line [plane ^ of demarcation between the two stars. This line is 

 formed of granules, which are a little larger than those of the rest of the 

 protoplasm. As the furrow deepens and surrounds the yolk, it assumes 

 a position oblique (in a constant direction) to the line joining the asters. 

 It is during the approach and mutual flattening of the cells that the new 

 nuclei appear at the centre of the protoplasmic part of each cell. Fol 

 further states ('75", p. 180) that he has never yet seen segmentation 

 preceded by a division, properly so called, of the nucleus, but would not 



* P. S. — Fol ('79, pp. 219, 220) has since corrected and explained the cause of 

 this error. 



