ON A CESTOBE FKO\r CESTI^ACTON. 411 



first. In fixed and stained preparations they become mnch 

 altered, having apparently become partly dissolved, and the 

 concentric lamination being no longer discernible, might very 

 easily be taken for nuclei. Like the oil globules, these bodies 

 consist, doubtless, of food materials ; but both these ingredients 

 of the yolk persist, not greatly diminished in bulk, to the 

 most advanced stage observed. Nothing was made out with 

 certainty as to the processes of maturation and impregnation. 

 The oosperm does not diifer to any appreciable extent from 

 the ovarian ovum. 



Very few, if any, unsegmented ova were found in the 

 secondary uterus. No definite history of the process of seg- 

 mentation could be traced, as there seemed to be great varia- 

 tion in the details. The first two segments (figs. 20 and 21) 

 are equal. One of these, or both, become divided into two 

 equal parts (figs. 22 and 23), and from the three or four 

 equal, or nearly equal cells thus formed, a number of smaller 

 cells become segmented off (figs. 24, 25, and 26). Eventually 

 the larger cells become reduced by division until a blastoderm 

 is formed consisting of a disc of small cells (figs. 27, 28, and 

 29), which are very irregular in size and shape, and present 

 no definite arrangement. This disc becomes thickened to 

 form a rounded mass, on the surface of which appears here 

 and there a flattened cell. In this stage there appears to be 

 no further cell-differentiation, except that there are present, 

 in the most advanced embryos, one or two pairs of very small 

 cells that become more intensely stained than the rest. It is 

 conjectured, from their arrangement in pairs, that these are 

 the cells destined to develop the hooks. 



No hooked embryos were found in the uterus of any of the 

 numerous specimens examined. But of a number of eggs 

 which had been kept in pure sea-water for five days, a large 

 proportion (figs. 30 — 32) were found to contain fully formed 

 active hexacanth embryos. It would thus appear that passage 

 to the exterior with the fiBces is, under normal circumstances, 

 the necessary condition for the development of the hooked 

 embryo. 



