Miscellaneous. 499 



division of the cells continuing in all directions, we thus find the 

 entoderm grow thicker and thicker. 



Between these two lamellae of entoderm and ectoderm forming 

 the ovary, there is a third, more delicate lamella destitute of struc- 

 ture ; this is the intermediate lamella, which separates them in a 

 very marked manner, and assists to define with certainty which 

 layer produces the ova of the Encope ; these ova always occurring 

 beneath the intermediate lamella, and being thus separated by that 

 lamella from the ectoderm, can only be developed from the ento- 

 derm. But another reason leads us to accept the cntodermic origin 

 of the ova of the Encope, if we observe directly all the graduated 

 transitions between the ordinary entodermic cells and the young 

 ova. The changes in an entodermic cell destined to be developed 

 into an ovum, which I must now notice, consist in the increase of 

 the volume of this cell and the transformation of the nucleus into 

 a germinal spot. 



In the entodermic cells lining the radial canals, the protoplasm 

 is perfectly transparent and destitute of granules ; the nucleus 

 appears in the form of a clear round spot, containing at the centre 

 a round nucleolus of greater density. Subsequently we observe 

 that the cells, as well as their nuclei and nucleoli, increase in size, 

 and the protoplasm becomes more and more granular. The nucleolus, 

 which is at first simple and furnished with a small vacuole, begins 

 to divide. As I have described in the case of a Medusa of the 

 White Sea *, at the commencement of the division the nucleolus 

 elongates, becomes constricted in the middle, makes a bend which 

 gives it the form of a horse-shoe, and finally divides into two parts, 

 each of which possesses a central vacuole ; then each half divides 

 again (simultaneously or not) into two parts, but in a direction 

 perpendicular to the first (as in the segmentation of the ovum), and 

 so on. 



Although these phenomena are constant and normal in the 

 Medusae of the "White Sea, I have only observed them exceptionally 

 in the Medusae of the Gulf of Naples. Usually in the latter the 

 division of the nucleus takes place in a perfectly different manner, 

 which has not yet been described. When, after it has become 

 elongated, the nucleolus presents a median constriction, it does not 

 divide into two parts, but simply becomes elongated in the form of 

 a band twisted upon itself; constrictions then forming at various 

 parts of it, the nucleolus, from being originally round, becomes a 

 long moniliform ribbon rolled up in several turns. Each division 

 of the chaplet is fusiform and round ; it regularly contains in the 

 middle a very small vacuole, and is united to the neighbouring 

 divisions by a thin and sometimes rather long articulation. Some- 

 times this long sinuous band, which reminds us of the nucleus of 

 certain Infusoria (Stentor, Spirostomum), splits into two bands. 

 Finally the grains or articulations of the chaplet separate, and, 



* u Studies on Ilydroida," Ann. & Mag. Nat. Hist. ser. 5. vol. i. p. 254, 

 pi. xiii. figs. 9-14 (1878). 



