48 COMPARATIVE ANATOMY 



recognition of a definite larval period which is followed by a period of 

 metamorphosis in course of which the adult characteristics are acquired. 

 A famous instance is that of the "glass-fish," a small transparent some- 

 what eel-like marine animal which was assigned to agenus,Leptocephalus, 

 and for many years failed of recognition as being the larva of common eels 

 of the teleost family, Anguillidae. 



The young fish produced by an egg endowed with a large yolk mass 

 attains a relatively large size before it is compelled to obtain food from 

 an external source. Differentiation tends to take place more slowly. 

 Prolongation of this period of functional immaturity and dependence 

 increases the need of protection. In oviparous sharks and skates this 

 need is met by the shell within which the developmental period is passed 

 and from which the well-grown young fish emerges when ready to begin 

 active and independent life. These shells must be sufficiently permeable 

 to allow oxygen to pass from the water through to the enclosed embryo 

 or young fish. 



The embryo and young of the viviparous fish not only receives maxi- 

 mum protection but may obtain from the mother some nutriment in 

 addition to the initial supply of yolk, and must somehow obtain necessary 

 oxygen from the maternal blood. In most viviparous sharks and skates 

 the developing young fish lies free in the posterior enlarged uterine region 

 of the oviduct. The walls of this part of the oviduct are highly vascular. 

 Even in the absence of any specialized mechanism for the purpose, it is 

 possible that nutritive materials may diffuse from the maternal blood and 

 pass to the embryo or young fish within, and there appears to be no other 

 way in which the developing fish can obtain its oxygen. In some sharks, 

 however, there is definite provision for transfer of material from mother 

 to young. From the inner surface of the wall of the oviduct develop 

 folds or processes (villi) which are richly vascular. Similar folds may 

 arise in the region of the abdominal wall of the embryo (in relation to the 

 yolk-sac: see page 53) and these two sets of projecting vascular structures, 

 maternal and embryonic, become closely approximated or interlocked in 

 such a way as to facilitate diffusion of materials from the blood of one to 

 that of the other. Respiration, a certain amount of nutrition, and 

 possibly to some extent the removal of excretion products from the 

 embryo are thus provided for. These sharks are sometimes referred to 

 as "placental" sharks because of the striking similarity of this arrange- 

 ment and the mammalian placenta. 



Viviparity in fishes is certainly, in an evolutionary sense, a secondary 

 rather than a primitive condition. This is attested by the fact, among 

 others, that in some viviparous Elasmobranchii the large egg is encased 

 within a shell whose vestigial character is indicated not so much by the 

 extreme thinness and delicacy of the shell as by the fact that it possesses 



