306 Thos, H. Montgomery. 



cells, around which are the large vitellocytes (Vit. C). But a more 

 interesting change is found in the egg, of which two transverse sec- 

 tions are deiiictcd in Figs. 37a and 37b. Fig. 37b is through the 

 gastrocoel. Fig. 37a, a few sections distant from the preceding, 

 shows just beneath the snperlicial cells of the germ disc a compact 

 group of from six to eight rounded cells {Mes. E., only four of them 

 visible in this section), which resemble the outer ectoblast cells. 

 These are evidently the earliest cells of the mesentoblast, because they 

 have the same situation and appearance as cells which later can be 

 rccog-nized with certainty as mesentoblast. But whence they origin- 

 ated I have not been able to determine, the question being whether 

 they are direct derivatives of the ectoblast or from some particular 

 invaginated cell of the anterior cumulus. The latter view would 

 seem the more probable, judging from their position within the 

 anterior cumulus. 



At the next stage seen, one of about 49 hours, both vitellocytes and 

 mesentoblastic cells have increased in number and come to occupy a 

 wider area. Fig. 40a, PL III, shows all the superficial nuclei of the 

 germ disc on ventral view, and they number 1441 ; at the posterior 

 cumulus {Cum. P.) the nuclei are larger because there the vitello- 

 cytes reach the surface. Fig. 40b is a drawing of the same egg, but 

 at a deeper focus, exhibiting only the nuclei of vitellocytes beneath the 

 superficial cells ; this figure demonstrates that the vitellocytes are 

 now scattered beneath the whole of the germ disc ; this figure does 

 not reproduce all of the vitellocytes, but only those whose nuclei could 

 by their superior size be readily distinguished from the nuclei of the 

 surface cells. Sections further illustrate this migration of vitello- 

 cytes; thus Figs. 41a, PI. Ill, and 42a, PI. IV, show their position 

 on median sections; Figs. 41b and 41c, PI. Ill, on longitudinal sec- 

 tions of the middle of the germ disc ; Fig. 43a, PL IV, on transverse 

 section of the disc anterior to the anterior cumulus ; and Fig. 43b on 

 cross section lateral to this cumulus. Accordingly, in this stage the 

 large, markedly branched and richly vacuolated vitellocytes are still 

 most abundant in the vicinity of the anterior cumulus, but many of 

 them have emigrated thence, some into the yolk, a greater number in 

 all directions beneath the germ disc, while there remains at the 



