304 Thos. H. Montgomery. 



There is to be noted particularly the origin of the posterior cumulus, 

 the rapid proliferation of vitellocytes and the segregation of the early 

 mesentoblast. 



On unstained germ discs examined in alcohol is to be found' behind 

 tJie anterior cumulus a second, smaller prominence, the posterior 

 cumulus (Cum. P, PI. II, Fig. 29; PL III, 32, 33; PL IV, 44). 

 This is variable in position, placed sometimes at the margin of the 

 germ disc, sometimes nearer the anterior cumulus (an extreme case 

 of which is shown in Fig. 32). Both cumuli are shown on profile on 

 a stained germ disc in Fig. 26, PL II. The two cumuli, at first gen- 

 erally se^Darated, become later connected by vitellocytes moving be- 

 tween them beneath the germ disc; this is well shown on a surface 

 view in Fig. 34, PL III, where the shaded portion marks the band 

 of vitellocytes. A line connecting the two cumuli marks the later 

 mid axis of the embryo, though, as we have seen, this could be fore- 

 told from the eccentric position of the anterior cumulus alone. 



The posterior cumulus difi'ers from the anterior, besides its later 

 development, in being a prominence from the start, in forming no 

 gastrocoel, and in producing only vitellocytes. But since it is a 

 region of inner cell proliferation it may well be considered one part 

 of a blastopore, the other part of which would be the anterior cumu- 

 lus; probably the blastopore was phyletically first a longitudinal 

 groove, the middle portion of which later disappeared. The earliest 

 stage found of the jDosterior cumulus, one of 30^/2 hours, is shown on 

 surface view in Fig. 26, PL II, and on median section in Fig. 27; it 

 is then composed of a few large cells ingesting yolk. Later stages 

 of it are illustrated in Figs. 30a, 36, 41a, PL III ; 42a, 42b, PL IV, 

 on median section, and in Figs. 35, PL III, and 43c, PL IV, on cross 

 section. Even in the late stage of Fig. 43c there are only about 

 twelve vitellocytes at the posterior cumulus, though these are unusu- 

 ally large; and Figs. 42a and 43c indicate that such cells are pro- 

 duced not only by invagination, but also by direct metamorphosis of 

 superficial cells of the germ disc at that point. It is by the presence 

 of this group of large yolk cells that one is enabled to identify the 

 position of the posterior cumulus with the posterior end of the embryo 

 of the succeeding protozonite stage. 



