318 FISHES 



The expulsion takes place very slowly, for the simple reason 

 that the egg has in the water no tendency to fall out under the 

 action of gravity, and since the secretory activity of the oviduct, 

 which may be considered to be excited by the presence of the 

 egg, diminishes gradually while the empty oviduct is collapsed 

 and thrown into folds, the secreted substance of the shell is 

 necessarily moulded to the cavity of the folded duct. The 

 secreted substance may, must indeed, be regarded as a plastic 

 substance which is being drawn through the folded oviduct by 

 the pressure of the water on the thicker portion of the shell 

 which has already escaped. Professor Dean describes a speci- 

 men in which the capsules were protruding : the posterior 

 narrow parts were still inserted in the oviducts and showed no 

 tendency to become detached. (Plate XXVI., C.) He admits 

 also that considerable time is taken in the process of egg-laying. 

 Similar conditions obtain in the case of oviparous Elasmo- 

 branchs, and the filaments in the egg-shell of Scyllium can be 

 explained in a similar way, at least the posterior ones which are 

 the last formed. The female Scyllium rubs her cloacal aperture 

 against upright objects at the bottom of the sea and the lower 

 tendrils which protrude first having become entangled pull first 

 the egg itself and then the upper tendrils from the oviduct ; the 

 latter are therefore pulled out from the secreted plastic substance 

 like a drawn wire. The same reasoning cannot, it is true, be 

 applied to the tendrils which are first formed since they are 

 completed before the main part of the shell is formed, but the 

 effect on the oviduct of the pulling out of the egg may well have 

 influenced its mode of action and shape at the beginning of the 

 secretion of the next egg. Thus instead of saying that the 

 tendrils are adapted to the future attachment of the egg after it 

 is laid, we may conclude that the action of the female in trying 

 to rid herself of the eggs has directly caused the development 

 of the tendrils. At any rate one explanation is as reasonable 

 as the other, unless it can be proved that conditions have no 

 permanent or hereditary effect, and this has not yet been 

 proved. 



No other fishes except the Elasmobranchs are provided with 

 a gland in the oviduct which secretes a true egg-shell. The 

 eggs when laid are therefore enclosed only by membranes 

 secreted in the ovary ; they are usually small and spherical and 



