The Development of Scyphomedusse. 241 



is composed of a dense mass of fine grains uniformly arranged, and 

 staining blue (hematoxjdin-eosin) . The nearly mature eggs (figs. 

 27-29) have an alveolar cytoplasm, the walls of the large alveoli 

 being made up of granules quite similar in size and staining reac- 

 tions to the grains present in the young oocytes. Within each, 

 alveolus is a more or less spherical body, not completely filling 

 the vacuole, though of varying size, which always stains with the 

 eosin. These are the yolk bodies and they are so abundant and 

 so large that they give to the egg substance a red color with the 

 stain used, the finer granules being hardly noticeable with low 

 magnification. 



Cleavage 



Although the cleavage of Aurelia was not followed in detail, it 

 appears to differ little if any, from that of Cyanea, and a similar 

 blastula is formed. Hyde (1894) found that the gastrulation 

 process was somewhat different in material from Maine and in that 

 from the Massachusetts coast. If the differences be correlated 

 with the environmental conditions of the regions, our material 

 (from Maine) should resemble that portion of Hyde's which came 

 from a similar locality. As a matter of fact, however, the gas- 

 trulation proves to resemble that of her Massachusetts mater- 

 ial, and confirms the results of Smith (1891) and our results on 

 Cyanea, all from the more southern material. 



The presence of cells free in the cleavage cavity was very rare 

 and from many different stages of gastrulation it is evident that in- 

 vagination is the chief, if not the only process. Fig. 30 shows an in- 

 vagination just beginning, only a small portion of the blastula 

 wall being involved. The invagination extends inward, assuming 

 a condition similar to that shown in fig. 31 ; this extension of the 

 infolded layer of cells being due in part to cell division and in 

 part to a change in the shape of the cells. The closure of the 

 blastopore, which soon takes place, begins inside and proceeds 

 outward (fig. 33). 



