198 bulletin: museum of comparative zoology. 



ever been seen. The supposed disappearance of the germinative 

 vesicle at this time, I believe to be due simply to the usual dissolution 

 of the nuclear membrane and the mingling of karyoplasm with cyto- 

 plasm which is characteristic of the prophase of mitosis. After this 

 dissolution of the nuclear membrane maturation spindles are formed, 

 two polar cells are constricted off from the egg, and the chromosomes 

 remaining in the egg form a definite vesicular egg nucleus. 



4. Nucleolus. — The nucleolus of oocytes of Hydromedusae has 

 been described as plasmatic and not aiding in the formation of chromo- 

 somes by Brauer ('9P), Hacker ('92), Morganstern (:01), Wulfert 

 (:02), Harm (:02), Trinci (:07), Muller (:08), and Conklin (:08). 

 Trinci (:05) finds in the Eucopidae that the nucleolus divides into pieces, 

 some of which are chromatic and resemble "pseudo-nucleoli" of 

 Amphibia; in Phialidium the same author (:07) describes a chief 

 nucleolus, with acid and basic constituents, not aiding in the formation 

 of chromosomes, and smaller chromatic nucleoli, which help to form 

 chromosomes. Stschelkanowzew (:06) maintains that in Cunina the 

 chief nucleolus alone, which is chromatic, forms the chromosomes, 

 though small plasmatic nucleoli are also present. Bigelow (:07) 

 describes the chief nucleolus in Gonionemus as staining like chromatin, 

 but it takes no part in chromosome formation and is considered as a 

 by-product; accessory nucleoli which occur are plasmatic bodies. The 

 nucleoli which do not form chromosomes are either (1) cast into the 

 cytoplasm, w^here they are dissolved, — Brauer ('91'^) Tubularia 

 mesembryanthemum, ('91^) Hydra (in part), Hacker ('92) Aecjuorea, 

 Morganstern (:01) Cordylophora, Harm (:02) Clava, Hargitt (:04''^) 

 Pachycordyle, (:06) Clava, Trinci (:07) Tiarella, — or (2) fragmenta- 

 tion occurs and the pieces are for the most part dissolved in the nucleus 

 — Brauer {'%V^) Hydra (in part), Wulfert (:02) Gonothyraea, Trinci 

 (:05) Eucopidae (in part), (:07) Phialidium, ■Nliiller (:08) Cladonem- 

 idae and Codonidae. 



In Pennaria and Tubularia crocea the nucleolus is entirely plasmatic 

 in all oogonia and oocytes. In Pennaria it seems gradually to dissolve 

 in the germinative vesicle, without fragmenting. In Tubularia it may 

 lose some of its substance in liquid form, or in fragments, at an early 

 and variable stage in the growth of the oocyte; but eventually the 

 remainder fragments into few or many parts. Of these, some may 

 dissolve in the nuclear sap; others are added to the nuclear reticulum 

 and are apparently transformed into chromatin. The complete dis- 

 appearance of the nucleolus within the germinative vesicle before the 

 dissolution of the nuclear membrane, and before the chromosomes are 



