82 GARDINER. [Vol. XV. 



which has not yet been drawn from the vitelline gland. Dr. 

 Sophie Pereyaslawzewa (3), in describing the young ovum of 

 what she terras the " Pseudoacoela," — practically the equiva- 

 lent of von Graff's Acoela, — speaks of fine grains of "vitellus 

 nutritif " adhering to the surface of the nucleus. The fate of 

 these grains she gives on page 149 as follows : " Cette dis- 

 position de toutes les parties inevitables de I'oeuf reste telle 

 jusqu'au moment dela fecundation, quand tout change : la vesi- 

 cule disparait et emporte avec elle la force de I'attraction qui 

 jusqu'a ce moment retenait les grains du jaune d'oeuf adherents 

 a la surface de le vesicule ; ils montent tous a la peripheric 

 de I'geuf, y stationnent, participent dans cette position a tous 

 les fractionnements de I'oeuf et restent inseparables du vitellus 

 formatif durant le d^veloppement embryonnaire. Comme il 

 reste inseparable du vitellus formatif, il est difficile de preciser 

 son role dans le developpement embryonnaire." I have never 

 observed either in the ova of P. candatus or that of any other 

 Turbellarian which I have studied any such changes as are here 

 described. 



The nucleus of the ovarian ovum is quite peculiar in its struc- 

 ture, and the change which it undergoes as the egg matures 

 is worthy of some attention. In the smallest germ cell it is a 

 clear structure with well-marked granular network e.xtending 

 from the nucleolus to the nuclear wall (Fig. 6). Within 

 the nucleolus is a spherical spot which stains very deeply. 

 This spot I have not been able to find in any but the ovarian 

 ova. Immediately after the ovum passes into the vitellarium 

 the whole nucleus increases enormously in size, so that its 

 diameter is quite as great as that of the ovarian ovum. This 

 increase is exceedingly rapid, for the intermediate stages are 

 seldom found. Still more striking are the changes which the 

 nucleolus undergoes (Fig. 7). Instead of a sphere, it grows to 

 be an enormous coil of densely staining substance, which forms 

 the most conspicuous feature in the whole ovum. The net- 

 work of chromatin in the nucleus (Fig. 6) gives place (Fig. 7) 

 to an exceedingly fine granular substance which stains less 

 deeply, and in which but faint traces of the formerly well- 

 marked network are to be seen. 



