OF THE HYDROMEDUSJE. 377 



ber and soon form a very thin layer of pavement epithelium, fig. 3, upon the outer sur- 

 face of the gelatinous umbrella, b. 



According to Fol, the gelatinous substance is not homogeneous, hut is marked by fine 

 striatums, which radiate through it in all directions from the surfaces of the endoderm 

 cells. The latter also increase in number, and hecome flattened as at c, in fig. 3, while 

 the digestive cavity, fig. 3d, becomes correspondingly enlarged. The endoderm 

 cells preserve their reticulated structure, which is visible until after the tentacles of the me- 

 dusa appear. When the gelatinous substance first appears, and for some time after, it 

 is uniformly thick and almost perfectly spherical, and theendodermal shell is also spherical, 

 concentric with the outer surface, and separated from the ectoderm at all points; hut it 

 soon approaches, and finally touches the ectoderm, at a point which is to become the 

 oral pole of the medusa, and which is below in fig. 3. The gelatinous substance, 

 which lies between the two layers, is ahsorhed at the oral pole during this process, and 

 Fol makes the very satisfactory conjecture, that the force which pushes the endodermal 

 sac to one side of the spherical embryo is produced by the more rapid secretion of the 

 gelatinous substance at the aboral than at the oral pole. The embryo now changes its 

 shape a little, and becomes slightly flattened at the ends of the principal or oral-aboral 

 axis, and the cells of both layers become thickened around the oral pole, to form an 

 oral area or peristome, a', c'. At this period the embryo rises from the bottom and 

 floats in the water, apparently at rest. Under the microscope, however, it is easy to see 

 that it does not simply float, but swims about with a very slow uniform motion, and al- 

 though I was not able to see any cilia, small floating particles were thrown away as if by 

 the action of cilia, which are undoubtedly present upon part if not the whole of the ec- 

 toderm. Fol states (22, page 482) that, at this stage in the development of the very 

 much larger embryo of Qeryonia fungiformis, scattered cilia make their appearance over 

 the whole ectoderm, and cilia are visible upon the oral area of our species at a later 

 stage, as shown at c, in figs. 4: and 5. 



The spherical larva, with its two concentric layers of cells separated from each other 

 by a gelatinous umbrella, without a mouth or any other passage into its spacious diges- 

 tive cavity, and swimming by means of ectodermal cilia, is at first sight very different 

 from the embryos of other medusa?.; but its peculiar appearance is due to the very early 

 formation of the gelatinous substance of the umbrella. If this Avere absent or if it made 

 its appearance at a later stage, the embryo would be a ciliated, mouthless, two-layered 

 planula, almost exactly like an ordinary planula, after the endoderm and digestive cavity 

 have made their appearance, but before the mouth has been formed. The Geryonidse 

 accordingly pass through a planula stage, directly comparable with the same stage in other 

 hydroids, but complicated by the accelerated development of the digestive cavity and 

 the gelatinous umbrella. 



The origin of the endoderm. at a very early stage of segmentation, by the simulta- 

 neous delamination of the inner ends of all the blastoderm cells is clearly a modification 

 of what occurs in ordinary hydroid planula 3 , although the segmentation cavity persists 

 as the digestive cavity, and the endoderm never forms a solid ma<s, as it certainly does 

 in the planuhe of Ilydractinia and Tubularia. In Eutima however the segmentation cav- 

 ity persists as it does in Liriope, and this is no doubt true also of other hydroids. 



