276 EMBRYOLOGY OF THE LOWEE VERTEBRATES oh. 



testis where a considerable length towards the posterior end may 

 lose its fertility and assume a merely conducting function, 1 or where 

 such sterile portions may be repeated at regular intervals throughout 

 the length of the testis (Gymnophiona). 



This concentration of the activity of the gonad may affect its 

 bilateral symmetry. In Elasmobranchs both ovaries may be present 

 and functional {Zaemargus,JVotidanus griseus), or one may be function- 

 ally inactive (Centrophorus, Trygon), or as happens in the majority, 

 one, usually the left, fails to complete its development and is reduced 

 to a more or less insignificant vestige. A similar reduction of one 

 ovary takes place in many Teleosts. In the Birds the right ovary 

 ceases its development at an early period and soon disappears entirely 

 in the majority of individuals, although exceptions are of com- 

 paratively frequent occurrence. 



Ovary and Oviduct of Teleostomatous Fishes. — In the 

 most archaic of existing teleostomatous Fishes — the Crossopterygian 

 ganoids — the ovary is in the form of a typical genital fold which 

 sheds its eggs into the splanchnocoele, from which in turn they pass 

 out by a Miillerian duct. Consequently we may take it, in the 

 absence of convincing evidence to the contrary, that the ancestral 

 condition of the ovary and oviduct in the teleostomatous Fishes did 

 not differ from that in other primitive gnathostomes such as Elasmo- 

 branchs, Dipnoans, or Urodeles. A peculiarity however of the 

 Crossopterygian oviduct as compared with that of the other groups 

 mentioned is seen in the reduction of its glandular activity, and this 

 reduction — which finds its physiological expression in the reduction 

 of tertiary egg envelopes — probably gives a clue to the subsequent 

 evolutionary history of the oviduct within the group Teleostomi, in 

 the majority of which the whole Miillerian duct has apparently been 

 reduced to the verge of complete disappearance. 



As regards the ovary itself there has come about secondarily — 

 perhaps in correlation with increase in number and diminution in 

 size of the eggs — a condition in which the eggs are not set free in the 

 general splanchnocoele but are shed into an ovarian cavity, the wall 

 of which is in complete continuity with that of the oviduct. The 

 ovarian cavity is formed by the walling in of the portion of 

 splanchnocoele which lies along the fertile (usually lateral) face of 

 the ovary. The precise method of enclosure differs in detail in 

 different Teleosts. In some {e.g. Perca, Gasterosteus, Acerina, Zoarces) 

 the ovigerous surface of the genital fold becomes invaginated, or 

 overgrown by flaps which eventually meet and undergo fusion 

 (Fig. 142, A): in others {e.g. Cyprinoids) the free edge of the 

 genital fold meets and undergoes fusion with the wall of the 

 splanchnocoele (Fig. 142, B). 



Which of these two types of development is the more nearly 

 primitive cannot be stated with certainty but the balance of 



1 Lepidosiren and Proto})terus (Graham Kerr, 1901) ; Polypterus and probably 

 Teleosts (see below). 



