316 FISHES 



jections of the vascular wall of the yolk-sac, penetrate into the 

 wall of the uterus and adhere to it, so as to form a true yolk- 

 sac placenta, through which nutrient material diffuses from the 

 maternal blood-vessels into those of the foetus. The existence 

 of this placenta in the smooth dog-fish of the Mediterranean, 

 Mustelus Levis, was known to Aristotle, but it is a curious fact 

 that in the species which occurs on the British coasts, Mustelus 

 vulgaris, no such placenta is formed. One species of the Elas- 

 mobranchs is exceptional in its mode of reproduction, namely 

 the Greenland shark, Lcemargus borealis, the eggs of which are 

 not enclosed in a shell formed by the oviduct and are fertilised 

 after being shed like those of Teleosteans. 



All the existing Chimaeroids resemble the oviparous Elas- 

 mobranchs in the character of their eggs and in the nature and 

 development of the egg-shell. The latter is elongated, and con- 

 sists of a narrow central portion in which the egg is contained, 

 and a flat flange on each side. (Plate XXVI., A and B.) The 

 cavity of the elongated capsule consists of three portions, the 

 widest portion a little behind the anterior end, elliptical in shape, 

 which contains the yolk of the undeveloped egg, an anterior por- 

 tion very short in Chimcera, and only a little narrower than the 

 first mentioned part, but narrower and longer in Callorhynchus, 

 and a posterior portion almost half the length of the whole 

 capsule and very narrow. The anterior portion is the first 

 formed, and is posterior in position when the egg is in the ovi- 

 duct ; when the embryo is developed this part contains the snout. 

 Owing to the close correspondence between the shapes and sizes 

 of these different parts of the egg-capsule and the parts of the 

 young fish which is developed in it, Professor Bashford Dean of 

 New York distinguishes them as snout-sheath, trunk-sheath, and 

 tail-sheath. The capsule is thus adapted in shape, not to the 

 egg as it exists when the capsule is formed, but to the embryo as 

 it afterwards develops. Still more remarkable are the adapta- 

 tions in the capsule for the admission of water for the respiration 

 of the embryo and for the escape of the latter when it is hatched. 

 Along the sides of the capsule are a series of apertures which are 

 not open when the egg is laid, but by a process of weathering or 

 decay become open in the later stages of development, and 

 allow of a current of water produced by the respiratory move- 

 ments of the embryo to enter by the anterior apertures and to 



