AMCEBOID MOVEMENTS OF THE YOLK. 505 



may sometimes be perceived, when sections are made, a 

 small cavity, corresponding in size to the formerly present 

 nucleus. According to Remak, this is the nuclear cavity 

 of v. Baer. This expression clearly indicates that on this 

 view the cavity occupied by the nucleus still remains when, 

 it has itself disappeared. 



If the fertilized germ be placed in favourable conditions, 

 a fresh nucleus speedily makes its appearance in its sub- 

 stance. In regard to this nucleus I can say nothing from 

 my own experience, and it would scarcely serve any good 

 purpose to adduce the concurrent testimony of others. As 

 the ova are usually opaque under these circumstances, it 

 is impossible to see the nucleus when fresh. If all observers 

 have been in such complete agreement in regard to the 

 formation of a new nucleus, this is essentially due to the 

 fact that in the later cleavage products of the germ the 

 nucleus is distinctly visible, whilst it may be shown in 

 addition that it is homogeneous, and has exactly the ap- 

 pearance of an oil drop. Reverting to the fact that the 

 old nucleus disappears, it is thus rendered highly probable 

 that we are here dealing with a new formation. 



Before the process of cleavage sets in in the germ, it 

 undergoes " certain automatic changes of form. The amoe- 

 boid movements of the Forella germ have already been de- 

 scribed (vol. ii., p. 185). If the freshly deposited ova of Bufo 

 cinereus be attentively watched, it may be seen that it 

 has at first several facettes, and that it subsequently as- 

 sumes a spherical shape. The occurrence of alterations of 

 form in the ovum of the Bird before the commencement 

 of cleavage, has only been deduced from a comparison in 

 hardened preparations of sections made through germs at 

 various stages of development. Here also attention may 

 be called to the observation first made by Bischoff in the 

 ovum of the Rabbit, that the yolk (germ, Keim) retracts 

 from the zona pellucida before it divides. 



Whether the observation in regard to the rotation of the unseg- 

 mented germ within the vitelline membrane, recorded in the same 

 place, should also be here mentioned as belonging to these movements, 



