MODES OF REPRODUCTION 335 



in all respects except size. The young fish at birth is •£ in. 

 long. 



In Anableps Wyman found that the number of embryos 

 varied from four to eighteen in a single female. The ovaries 

 were united into a single sac into which the follicles containing 

 the embryos projected. In this species the yolk was soon 

 absorbed, and after the first stages the umbilical sac contained 

 no yolk, although it was of large size ; it was occupied by 

 coils of the intestine. The external surface of the sac was 

 covered with a number of parallel series of projecting processes 

 which followed the course of the blood-vessels in the wall of the 

 sac. Evidently here we have a more modified condition than 

 in Gambusia : the embryo is no longer nourished entirely by 

 the yolk but the papillae on the walls of the sac have been 

 evolved for the purpose of absorbing nourishment from the 

 follicles ; the latter contain a nutritious liquid secreted by their 

 walls, which are richly supplied with blood-vessels. This 

 adaptation is closely similar to that which is found in Mustelus 

 among the Elasmobranchs and to what is called the yolk-sac 

 placenta in mammals. In one specimen of Anableps were 

 found seven embryos 2\ in. in length whose development 

 was nearly completed ; the yolk-sac had disappeared and 

 was only represented by a thin membrane on the lower side 

 of the abdomen, on which no trace of the absorbing papillae 

 of the earlier stages remained. The embryos or young fish 

 had escaped from the follicular sacs and were lying free in the 

 cavity of the ovary ; evidently they were almost ready for birth. 

 In these embryos of various stages of development could be 

 traced in a most interesting way the gradual development of 

 the extraordinary horizontal division of the eyes which is 

 peculiar to Anableps : in the youngest embryos the eyes were 

 simple as in ordinary fishes, in later stages the iris developed 

 lateral projections, and finally the cornea was divided by a 

 horizontal band. The details and the significance of this modi- 

 fication of the eye are discussed in another part of this volume. 

 The Embiotocidae belong to the division Perciformes of the 

 Acanthopterygii, and accordingly in general characters they 

 are not unlike the common perch. They have a single elong- 

 ated dorsal fin, of which the anterior part is spiny, the posterior 

 soft or flexible. It is a curious fact that although copulation 



