458 BULLETIN OF THE 



or less essential part of the nucleus is removed in the formation of the polar 

 cells ; so that in cases, e. g. Arthropoda and Rotifer a, where polar cells are 

 not formed, and an essential part of the nucleus not therefore removed, par- 

 thenogenesis can much more easily occur than when polar globules are 

 formed.''^ 



" It is possible," Balfour further observes, " that the removal of part 

 of the protoplasm of the egg in the formation of the polar cells may be a 

 secondary process due to an attractive influence of the nucleus on the 

 cell protoplasm, such as is ordinarily observed in cell division." 



Repiachoff ('78, p. 412, Figs. 1-10) gives a brief account of the 

 structure of the germinative vesicle and some of the changes which 

 overtake it in the case of Tendra zostericola, but reserves an extended 

 account for a future occasion, when his observations shall have been 

 concluded. In the black, round ovarian eggs the vesicle possesses a dis- 

 tinct membrane ; the germinative dot is of irregular form and embraces 

 several irregular vacuoles. When the egg has assumed its peculiar bi- 

 lateral form the vesicle still retains its membrane, and there is then to 

 be found in stained eggs within the germinative vesicle a single, or 

 sometimes two nucleoli, and other spots less deeply stained than the 

 nucleoli, but more deeply than the nuclear fluid. Sometimes it was 

 impossible to find evidence of the existence of a germinative dot in any 

 form. He only hints at the possible fate of the vesicle, and then calls 

 attention to the existence of two polar bodies (" Excretkorperchen '"?) 

 difl'ering considerably in size, which were observed in the plane of, and 

 just prior to, the first segmentation. 



The peculiar growth and activity of the egg of Toxopneustes variega- 

 tus has already been given. Selenka (78") adds in the present paper 

 that in the germinative dot there arise vacuoles, which appear to lead to 

 its complete dissolution ; of this, however, he is made doubtful by the 

 different results obtained by 0. Hertwig. He is in accord with Fol and 

 Hertwig as regards the formation of polar globules by the division of a 

 spindle and the re-formation into the " Eikern " of so much of the latter 

 as remains in the yolk. It is perhaps doubtful if ''pronucleus" is in a 

 morphological sense a proper expression, since neither sperm nucleus 

 nor egg nucleus can alone play the role of a cell nucleus. 



While the polar globules emerge, a drop af protoplasm free from 

 granules flows out and soon envelops the whole yolk in the form of a 

 cortical layer endowed with automatic motion. Its fate is threefold : 

 (1.) its outer limiting layer is afterwards elevated as a vitelline mem- 

 brane ; (2.) a part penetrates with the spermatozoon into the "clear 



