DEVELOPMENT 325 



case of small pelagic ova, the amount of yolk available for the 

 nutrition of the embryos is comparatixely small, and conse- 

 quently only suffices to support embryonic development for a 

 short time. 



In viviparous fishes the development is necessarily almost 

 wholly "embryonic." The majority of Selachians produce their 

 young alive, and of the order of Rays {Hypotremata) only the 

 true Skates and Rays (Raiidae) and a few allied forms are 

 oviparous. The number of young produced at a birth varies. 

 Thus, the Spiny Dog-fish (Squalus) produces only 7 or 8, the 

 Monk-fish (Squatina) 25, and as many as 32 have been counted 

 in a single Tope (Eugaleus). In nearly all vi\'iparous Selachians 

 some sort of connection is established between the growing 

 embryo and its mother, serving to aid its respiration and 

 nutrition, and in its more elaborate forms recalls the com- 

 plicated placenta developed in mammals. During the early 

 stages of development the embryo is nourished by the yolky 

 portion of the ovum, which, after a time, comes to lie in a flask- 

 like bag or yolk-sac, attached to the under surface of the body 

 of the embryo by a long and narrow neck (Fig. 117). When first 

 formed the contents of the yolk-sac pass directly into the 

 alimentary canal, but at a later stage the connection between 

 the two is interrupted and the last remains of the yolk are 

 absorbed by the blood-vessels alone. In many Selachians the 

 walls of the lower part of the oviduct {i.e. the uterus), in 

 which the embryos develop, throw out long filamentous processes 

 known as uterine villi or trophonemata. These are richly 

 supplied with tinv blood-vessels and secrete a creamy nutritive 

 fluid, which is either absorbed by the blood-vessels of the 

 embryonic yolk-sac, and thus passed to the embryo itself, or is 

 taken up in a more direct manner. In some of the tropical 

 Sting Rays ( Trygonidae) and Eagle Rays {Myliobatidae) the fluid 

 seems to be taken into the alimentary canal of the embryo 

 directly through the mouth or spiracles. In the Butterfly Rays 

 (Pteroplatea) the villi from the walls of the uterus are particularly 

 long, and these are gathered into two bundles which pass 

 through the very large spiracles into the pharynx of the embryos, 

 of which there may be one to three, and it is probable that the 

 secretion is first digested in the alimentary canal and afterwards 

 taken up in the embryonic blood-vessels. In a few Sharks such 

 as certain of the Smooth Hounds {Mustelus) another type of 

 connection is established between embryo and parent when the 

 food material in the yolk-sac is nearly used up. The walls of the 



