182 



LECTURE IX. 



I have selected the successive two-fold {fig. 81.), four-fold (fig. 

 82.), and eight-fold {fig. 83.) generation of germ-cells. As the fis- 

 sures become more numerous, they take a diverging or radiating 



80 81 82 83 



Cyanaea aurita. 



course, until the whole surface of the ovum presents a granulated cha- 

 racter. And now the ovum loses its violet colour and transparency 

 and becomes a dark yellow ; it is covered by an epithelium, with 

 traces of cilia ; these increase, and at length cover the whole sui'face, 

 and the new being passes from the condition of an ovum to that of 

 the embryo and obeys the involuntary moving powers. A cavity next 

 begins to be developed in the centre of the germ-cells, which, by 

 continued and reiterated division, take on the form of truncated 

 pyramids, converging towards that centre {fig. 84.). 



The deep yellow ciliated embryo of the Medusa, which has now a 

 diameter of one eighth of a line, exchanges its rounded for an oval 

 form. It gradually elongates, and takes on a leucophrys-like form, 

 being half a line to one-third of a line in length. At the upper end is a 

 fossa, which does not, however, communicate with the central cavity, 

 and is not a mouth. This stage of development occupies two or three 

 days, when the ciliated monadiform embryo {fig- 85.), quits the 

 maternal pouch, and swims forth : the arrows indicate the direction of 

 the ciliary currents. The liberated and locomotive larvae sometimes 

 re-enter the generative cavity and get entangled between the folds and 

 tentacles of the ovarium, which led Ehrenberg to describe them as 

 ovarian ova ; but Siebold observes, that if they were produced there as 

 gemmules with the power of swimming, the marsupial sacs, in which 

 they actually acquire that development, might have been dispensed with. 



The great or cephalic end next becomes 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 

 developed into four processes, richly furnished with vibratile cilia 

 {fig. 86.). 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 is effected. 



The young Medusa, having swam through its infusorial stages, 

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

 phosis : during which the yellow colour disappears ; and the body 

 becomes subtransparent ; it also manifests a much more general irri- 



