254 BULLETIN OF THE 



of a vesicular nature. In Tropidonotus natrix the centre of each such vesicular 

 space is occupied by a " Schron's grain," and in this are embraced a number of 

 fine granules (Keinipiinktchen). 



Kleinenberg ('72, p. 41, Taf. II. Fig. 12) was probably the first to call at- 

 tention to a differentiation of the contents of the germinative vesicle into " a 

 viscid, plasmoid, filamentous mass and a more fluid substance." 



In his paper on Hydra, it is shown how, by a process of vacuolation, the 

 contents of the germinative vesicle are separated into a continuous thin layer, 

 lining the nuclear membrane, and a thicker mass concentrated about the ger- 

 minative dot. The intervening space is tilled by a fluid clear as water. The 

 latter is traversed, however, by numerous delicate filaments, which connect the 

 peripheral and central accumulations of granular protoplasm. 



Since the appearance of this paper, the reticular nature of the germinative 

 vesicle has often been observed ; for example, by Flemming ('75, p. 100, Taf. 

 I. Figs. 14-20) in fresh eggs of Unio ; 0. Ilertwig ('75, pp. 351, 352, Taf. X. 

 Fig. 1, Taf. XI. Fig. 9) in fresh eggs of Toxopneustes and in the eggs of 

 the mouse ; Ed. van Beneden ('75, p. 690, '76«, p. 64, Fig. 9, '76*, p. 170, PI. 

 XIII. Fig. 9) in the rabbit and Asteracanthion ; * Biitschli ('76, p. 218, Taf. I. 

 Figs. 6-8) in Nephelis ; K. Hertwig ('76", p. 77, Taf. III. Figs. 8, 9) in the sea- 

 urchin and the frog ; Giard ('77« p. 720, '77<^, p. 434) in Echinus miliaris ; Fol 

 ('77'^, p. 440) in Asterias, Sagitta, etc. ; Hoffmann ('77«, p. 33) and Whitman 

 ('78«, pp. 13, 14, PI. XIII. Fig. 61) in Clepsine; and Balfour ('78^ pp. 412, 418, 

 437) in Scyllium. 



Beside an intranuclear network of finely granular pale substance in the 

 germinative vesicle of Unio and Anodonta, Flemming ('75, pp. 95-105) dis- 

 covered that eggs which have attained a certain stage of development embrace 

 in their nuclei not a single, but two apposed nucleoli. These differ in their 

 reactions and in size. The one, which is called the principal nucleolus (Haupt- 

 theil), resists more the action of acetic acid, and is stained more deeply than 

 any other part of the vesicle. The other, which is called the accessory part 

 (Nebennucleolus), exhibits the same reaction as do large numbers of smaller 

 nucleolar structures which are distributed through the network and which vary 

 greatly in size. They are all more readily affected (swollen) by acetic acid, 

 and are less intensely stained than the Haupttheil. The nuclear contents are 

 still less stained, and the intranuclear cords become in stained objects invisible. 

 The accessory nucleolus, at first absent, is, during early stages in the growth of 

 the egg, smaller than the principal nucleolus, but ultimately exceeds the latter 

 in size. Flemming is inclined to regard the accessory part as a product of the 

 main nucleolus (constantes Quellungsproduct) but not as resulting from divis- 

 ion. For this reason the multinucleolar condition of a nucleus may be regarded 

 in a different light from that in which Auerbach ('74) has considered it, 



Trinchese ('76) was the first, I believe, to call attention to the reticulum 

 in the germinative vesicle of the (immature) human ovum. He characterizes 



* This network is the nucleoplasm of Van Beneden. 



