no 



LECTURE IX. 



scribed and figured by Dr. Siebold, from whose Memoir I have selected 

 the four-fold {fig. 58.) and the eight-fold {fig. 59.) generation of 

 yolklets represented in the diagrams. These are progressively multi- 

 plied by fissures, which are represented as proceeding by a diverging or 

 radiating course from the centre, until the whole surface of the vitelline 

 mass presents a granulated character. And now the ovum loses its 

 violet colour and transparency, and becomes a dark yellow. The mem- 

 brana vitelli acquires an epithelium {fig. 60, a) of the same colour, on 

 which traces of cilia are perceptible. These at length cover the whole 

 ovum, which may then be said to have taken on its embryonic 

 state. A cavity (6) is next observed to be developed in the centre 

 of this yellow-coloured ciliated gemmule. It rapidly changes the 

 round for the oval form, and then becomes elongated like the infu- 

 sorial Leucophrys {fig. 61.). In this state it quits the maternal pouch, 

 and swims with the great end foremost. The liberated and locomotive 

 young Medusse sometimes re-enter the generative cavity, and get en- 

 tangled between the folds and tentacles of the ovarium, which led 

 Ehrenberg to describe them as ovarian ova ; but Dr. Siebold observes, 

 that if they were produced there as gemmules with the power of 

 swimming, the marsupial sacs, in which they actually acquire that de- 

 velopment, might have been dispensed with. 



The young Medusa, having swam through its polygastric stage, 

 attaches itself to some firm body, preparatory to its next metamor- 

 phosis. The great or cephalic end is shortened and thickened, and a 

 depression is observed in its centre, which is the commencement of a 

 digestive cavity ; then the margin of this cavity expands, and is deve- 

 loped into four processes, richly furnished with vibratile cilia {fig. 62.). 

 A small cavity or disc for adhesion is formed at the opposite extremity 

 of the body, and thus the metamorphosis from the polygastric to the 

 rotiferous form of infusory is effected in the embryo Medusa. During 

 these changes the yellow colour is lost, and the body becomes 



colourless and transparent ; it 

 also manifests a more general 

 irritability, sometimes elon- 

 gating, sometimes contracting 

 itself. 



Four other ciliated cephalic 

 processes or arms next ap- 

 pear in the interspaces of the 

 first four, and all increase in 

 length ; these eight arms have 

 the power of remarkably short- 

 cyanaea. cuing and elongating them- 



