DEVELOPMENT. 85 



2. Development of the Ovum. — Embri/onal Developmenf. 



The general form and strnctnrc of the ovum has already been considered ; the phenomena 

 presented by the development of the embryo now remain for discussion. 



Devehjmient of the Embryo from the commencement of the segmentation of the Vitellus to the 

 attainment of the free locomotive stage. — I shall here describe this process as 1 have observed it in 

 Laomedea fexiiom, which may be regarded as affording a type of cmliryonal development through- 

 out the IIydroida. In this species the gonophores, which belong to the adelocodonic class, are 

 included 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 sup- 

 porting column (woodcut, fig. 18). Each gonophore in the female colony contains but a single 

 ovum — a fact which facilitates the observation of the development. 



The mature-ovum (woodcut, fig. 39 A), previous to the commencement of segmentation, is 

 about 001 inch in diameter; it is of a granular structure, and contains a very distinct cldar 

 germinal vesicle about 0002 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 or oval bodies, which lie 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, how- 

 ever, a little tincture of iodine be previously added to the water, they continue visible, and are 

 now plainly seen to be themselves vesicles, containing within them a few minute granides which 

 have been rendered obvious by the action of the iodine. 



The vitellus is entirely composed of minute spherical corpuscles of apparently homogeneous 

 structure, about 00002 of an inch in diameter, along with granules so small as not to admit of 

 measurement. There is no obvious vitellary membrane in the mature ovum, but I have satisfied 

 myself of its presence while the ovum is still in a very young state. In other species, Ilydractinia 

 echinata for example, this membrane is very obvious in the ovum just before segmentation. 

 There is no trace of a micropyle in the ovum of this or of any other hydroid which I 

 have examined. 



As already said, there is never more than a single ovum in each gonophore of Laomedea 

 flexuosa ; and as this ovum continues to enlarge, it presses back the spadix until the latter is 

 reduced to a small hollow projection in the bottom of the gonophore. 



Up to this time the germinal vesicle continues quite distinct, but it now entirely disappears 

 (B). The disappearance of the germinal vesicle is unaccompanied by any apparent change 

 in the structure of the ovum, which retains the same peculiar composition of spherical cor- 

 puscles and granules. I have little doubt that the vesicle now ceases to exist, and that its 

 disappearance is not due to its being merely concealed in the mass of the vitellus. It has 

 probably burst, and in so doing must have liberated its peculiar contents, which will then, of 

 course, be no longer visible in the vitellus. The disappearance of the germinal vesicle is 



