BIGELOW: NUCLEAR CYCLE OF GONIONEMUS MUltBACIIII. 293 



differentiation was shortly afterwards confirmed by Korotneff ('76) for 

 Lucernaria, and by 0. Hertwig ('78 b ) for various medusae and siphono- 

 phores. This latter author, who was the first to direct especial attention 

 to the oocyte nucleolus, found that this structure also was often com- 

 posed of two different substances, an outer layer and a more transparent 

 central mass. 



Most medusae, according to his observation, have only a single 

 nucleolus. But to this rule an exception is found in Eucope, in which, 

 although the young oocytes have only one, more advanced stages contain 

 several. Nussbaum ('87) and Brauer ('9l a ) have extended the latter 

 observation to Hydra, finding that the germinative vesicle at first con- 

 tains only one large nucleolus, and that with growth from two to five 

 small ones appear ; and Fiedler ('88) has made similar observations on 

 Spongilla. 



These studies on the nucleolus are reviewed in detail by Montgomery 

 ('98 b ), who has added a description of the germinative vesicle of a 

 siphonophore, perhaps Rodalia, which contains one large vacuolate nu- 

 cleolus, and several smaller homogeneous ones, with a different staining 

 reaction. Two different kinds of nucleoli have more recently been de- 

 scribed by Conant ('98), and by Morgenstein (:0l). 



Finally I should mention the studies of Doflein ('96) on the changes 

 which take place in the staining reactions of the various nuclear struc- 

 tures during the growth of the germinative vesicle of Tubularia. 



4. Maturation, Fertilization, and Early Cleavage. 



The papers which relate to maturation, fertilization, and cleavage in 

 coelenterates fall into two groups, according as they do not or do take 

 cognizance of the chromosomes. In the first group are the works of 

 Kleinenberg ('72), Fol ('73), Metschnikoff ('74), Korotneff ('76), 0. Hert- 

 wig ('78 b ), Claus ('82), Kowalevsky et Marion ('83), Metschnikoff ('86), 

 von Koch ('87), Fiedler ('88), and Hickson ('88). These, in the light 

 of our present knowledge of nuclear phenomena, are now of little more 

 than historic interest. Among them, however, should be especially 

 mentioned, for their importance in the general advance of cytology, 

 Fol's ('73) discovery of the double stellate figure in the egg of Geryonia, 

 and 0. Hertwig's ('78 b ) studies on the role of the germ nuclei, and on 

 their relation to the cleavage nuclei in Mitrocoma. Earliest in the sec- 

 ond group is Boveri's ('90) study on the maturation, fertilization, and 

 early cleavage of the medusa Tiara. In this species Boveri was able to 



