94 THE MEDUSAE. 



large, its cytoplasm dense and homogeneous. The outer cell, which, follow- 

 ing Metschnikoff, we may call tlie nurse-cell, and which appears to have 

 only a nutritive function, has vacuolated cytoplasm. The boundaries of the 

 two cells are easily distinguished, the sharpness of limitation in the figures 

 being no exaggeration of that in the specimens. The inner cell divides by 

 mitosis (PI. 22, fig. 13) ; two and four-cell stages were frequently observed 

 (PI. 22, fig. li ; PL 23, figs. 1, 2) ; and through successive divisions a typical 

 morula is formed (PL 23, figs. 5-7). The cleavage closely resembles the pro- 

 cess as described by Metschnikoff, except that he found no stages in mitosis. 

 Division of the cells is complete, and the boundaries of the blastomeres dis- 

 tinct and easily seen. Stchelkanowzew describes the cleavage of Cunina as 

 superficial, the nuclei alone dividing, and thus forming a syncytium ; but his 

 figures suggest that his material was not very satisfactory, so that in view of 

 the close correspondence of the present material with Metschnikoff's account, 

 I am inclined to believe that cleavage is normally total. During all these 

 stages the nurse-cell persists, and in all favorable examples its cytoplasmic 

 body can easily be distinguished by its vacuolated nature and clearly visible 

 outline from the cells of the larva which it encloses. The nucleus of the 

 nurse-cell grows until its diameter is two or three times that of the nuclei 

 of the morula, and becomes flattened until it forms a sort of cap to the 

 latter (PL 23, figs. 6, 7). It may remain single, or may divide directly 

 once or twice, and there is some evidence that one or more of the nuclei 

 resulting from such division may degenerate. This history of the nurse-cell 

 agrees closely with Metschnikoff's account. Stchelkanowzew, however, could 

 find no such structure in the early stages, while in the morula stage he 

 observed two nurse-cells (: 06, taf. 30, figs. 32, 33, 35, n, n'), which he be- 

 lieves are modified ectoderm cells of the larva, situated at the two poles of 

 the latter. According to his view, the cells which Metschnikoff described as 

 nurse-cells in early cleavage stages are nothing more than blastomeres. 

 I feel certain that Metschnikoff's interpretation is the correct one, at least for 

 the present species, since the history of the nuclei as well as the structure 

 of the cytoplasm clearly distinguishes nurse-cell from blastomere ; while 

 the preparations are so clear as to leave no doubt that the former encloses 

 the latter. 



The embryo up to now lies within the jelly, close to the endoderm layer. 

 Later stages, however, are to be seen only in the gastric cavity, or in the 

 peripheral canals. The nurse-cell is apparently destroyed during the passage 



