ON THE REPRODUCTIVE SYSTEM IN THE HYDROIDA. 403 
generative elements are externally confined by an ectodermal covering, which 
is in the same way the homologue of the endotheca in an ordinary gonophore, 
but here flattened“out like the spadix, in accordance with the ribbon-shaped 
form of the gonophore. 
he Multiplication by Fission. 
Kolliker* observed a process of true fissiparous multiplication in a medusa 
(Stomobrachium mirabile, Koll.) obtained in abundance at Messina. The fission 
always commenced by a vertical division of the manubrium, which thus became 
doubled ; and this stage of the process was followed by a similar division of 
the umbrella, separating the animal into two independent halves. The process, 
however, did not stop here, but was followed by a further division of each of 
the two first-formed segments into two others, by a fission at right angles to 
the direction of the first ; while Kolliker’s observations led him still further 
to conclude that the process does not terminate with even the second cleavage, 
but, on the contrary, that it still goes on, the animal continuing to multiply 
itself by frequent acts of fission. 
Developed generative bodies were not observed in Stomobrachium mirabile, 
and Kolliker is of opinion that this medusa is only the young of another 
(Mesonema cerulescens, Koll.) found in the same seas, and in which no divi- 
sion takes place, but in which well-developed generative sacs occur along the 
course of the radiating canals. 
B. Sevual Reproduction.— Generation. 
The origin and mode of formation of the ova and spermatozoa have already 
been considered ; the phenomena presented by the development of the embryo 
now remain for discussion. 
Development of the Embryo, from the commencement of the segmentation of 
the Vitellus to the attainment of the free locomotive stage——I shall here de- 
seribe this process as I have observed it in Laomedea flexuosa. In this 
species the gonophores, which belong to the adelocodonic class, are in- 
cluded within a gonangium, where they are borne along the whole length 
of a blastostyle, regularly increasing in maturity as they recede from the 
base towards the summit of their supporting column. Each gonophore in 
the female colony contains but a single ovum—a fact which facilitates the 
observation of the development. 
The mature ovum (fig. 20 A), previous to the commencement of segmenta- 
tion, is about 0-01 inch in diameter ; it is of a granular structure, and contains 
a very distinct clear germinal vesicle about 0-002 of an inch in diameter, 
situated very excentrically, and easily separated from the surrounding vitellus, 
when it may be isolated as a perfectly spherical vesicle upon the stage of the 
microscope. There is occasionally a single germinal spot, but its place is 
usually taken by several (2 to 10) minute more or less spherical bodies, 
which float in the perfectly transparent and colourless fluid contents of the 
germinal vesicle. When the germinal vesicle is freed from the surrounding 
vitellus and floated in sea-water on the stage of the microscope, these bodies 
almost instantly disappear without leaving a trace behind, being apparently 
dissolved by water absorbed from without through the walls of the vesicle. 
If, however, a little tincture of iodine be previously added to the water, they 
continue visible, and are now plainly seen to be themselves vesicles, containing 
4, 
* Zeit. f. wissen. Zool. 1853, p. 352. 
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