280 BULLETIN OF THE 



It soon disappears entirely, but immediate treatment with acetic acid 

 brings again to view what remains of it, — only a trace (Andeutung) of 

 the former nucleus, — and at the same time two accumulations of pro- 

 toplasm, whose closely massed granules assume the form of two regular 

 stellate figures, one on either side of the remnant of the vesicle. The 

 rays of these stars are formed by the granules, which are arranged in 

 straight lines. Many such lines stretch from one star-centre to the 

 other in an arch, thus embracing the remnant of the " germinative vesi- 

 cle." A little later the acid fails to disclose a trace of the nucleus, but 

 the stellate figures are unchanged save that they are farther apart.* 

 By the segmentation of the yolk these centres of attraction become more 

 and more separated, and there now appear in each of them one, two, 

 three, up to eight or ten small vacuoles, which ultimately melt together 

 and become so rounded as to present exactly the same appearance as the 

 undivided "germinative vesicle." 



Such is the formation of the new nuclei. These, with similar obser- 

 vations on mollusks, worms, etc., lead Fol (p. 487) "to accept in full 

 Sachs's theory of segmentation by Anziehungs-Mittelpuncte." "At seg- 

 mentation the 'germinative vesicle' every time disappears, and in its 

 place there arise in the protoplasm two centres of attraction, in which 

 latter the new nuclei appear " (p. 486). 



BiJTSCHLi's ('73«, pp. 101-104, Taf. XXVI. Figs. 6P, 61^) studies 

 were made on the egg of Rhabditis dolichura, Schneider, without the 

 use of reagents. They really cover earlier stages in the ontogeny than 

 the observations of Fol, inasmuch as Biitschli observed the approach and 

 contact of the two structures which we now know to be the pronuclei. 

 When the latter have reached the centre of the egg, they appear almost 

 as though melted together, and the yolk granules become suddenly 

 grouped radially to the body thus formed. The latter lengthens in the 

 direction of the long axis of the egg, and assumes a lemon shape. After 

 some time one observes at either pole of this figure a knoblike protuber- 



* When Auerbach {'74, p. 254) intimates that Fol's account is not quite clear, 

 one has no opportunity to object ; but when he makes Fol responsible for the [im- 

 plied ?] statement that the nuclei of the first segmentation spheres are produced by a 

 division of the germinative vesicle, and that the radial figures appear for the first time 

 in the stages preparatory to the second segmentation, then it must be objected that 

 the less plausible of two explanations is the one put forward. At least, I see no 

 reason why Fol may not haVe called attention to a figure (Fig. 2) representing a cor- 

 responding stage in a later segmentation (just as Auerbach himself, p. 225, has done) 

 to illustrate the phenomena of the first segmentation, without involving himself in 

 the inconsistency with which he has been charged. 



