142 Memoir Sears Foundation for Marine Research 



bling keratin'* and constructed of several fibrous layers, becomes more brittle on ex- 

 posure to sea water. The body of the capsule is quadrate in shape and the four corners 

 are drawn out in long tubular horns, which taper to filamentous tips in some species 

 but which are blunter in others, the pair at one end being considerably longer than 

 those at the other. As a rule each horn has a slit-like opening on the outer side which 

 is closed in early stages either by a plug of albumen or by a delicate membrane. Later 

 the albumen is absorbed, leaving the slits open. Also, in some species the lateral margins 

 of the capsule are keeled or fringed, and in most species (perhaps all) the margins, 

 sometimes the sides as well, bear series of silky and sticky filaments; these mat into 

 felt-like masses that serve to anchor the egg to algae, pebbles or other objects on the 

 bottom. Before they are laid, the egg capsules lie with the longer pair of horns directed 

 toward the cloaca and with the more convex face of the capsule toward the dorsal side 

 of the mother. 



It appears that osmosis, which takes place through the shell and by which equi- 

 librium is maintained with the surrounding sea water" provides the necessary oxygen 

 for the embryo at first. The means by which the aeration of the embryo is accomplished 

 after the albumen has been absorbed has been the subject of controversy. However, 

 recent experiments seem to have confirmed the old view that sea water passes in and 

 out through the slits on the horns of the capsule after they have opened, thus sup- 

 plementing the effects of osmosis through the shell in providing the necessary intake 

 of oxygen and outgo of carbon dioxide.'* 



Observations in aquaria have shown that the eggs of at least some species are 

 laid in pairs, one soon after the other, with a rest period of one to five days before the 

 deposition of another pair. A single female may produce eggs during a period of several 

 months, in spring, summer, or fall. For example, one female of the European Raja 

 brachyura Lafont 1873 ^^^^ 25 capsules at irregular intervals during a 49-day period 

 (April 12 to May 31) in the aquarium at Plymouth, England. It has been observed 

 also that the egg capsules in aquaria are buried in the sand. In most cases the capsules 

 that have been dredged from the sea bottom have been firmly fastened to masses of 

 broken shells, fragments of algae, gravel, sand, etc., and Skate eggs in the Plymouth 

 Aquarium have been seen to adhere to objects of this sort immediately after they were 

 laid. Under aquarium conditions, the incubation period for six common European 

 species averaged from 4V2 to about 14V4 months. 



Normally, the embryo lies within the capsule with its head pointing diagonally 

 toward the longer pair of horns, its pectorals folded over onto the dorsal surface and 

 its tail curled forward over its back. At hatching it emerges through a transverse slit 

 between the two longer horns,'^ and the empty capsules, popularly called mermaids' 

 or sailors' purses, are familiar objects along the seashore. 



36. Its chemical composition was determined by Hussak and Welker (J. biol. Chem., 4, 1908: XLIV-XLV). 



37. This maintenance of equilibrium was demonstrated by Peyrega (Bull. Soc. zool. Fr., Jp, 1914: 211). 



38. See Clark (J. Mar. biol. Ass. U. K., 12, 1922: 5S4) for an account of these experiments, with summary of older 

 observations on the aeration of the Skate embryo. 



39. See Clark (J. Mar. biol. Ass. U. K., 12, 1922: 582-591) for an interesting account of the egg capsule, egg laying, 



