1060 



SUBMAMMALIAN VERTEBRATES 



This may be compared to the observation 

 that progestin can be detected in tlie blood 

 of roosters but not of capons (Fraps, 

 Hooker and Forbes, 1949). 



XXI. Developmental Basis for 

 Sexual Diniorphisni 



Fish 



Most embryologists believe that in am- 

 phibians and amniotes the functional ovar- 

 ian cortex develops from the primitive cor- 

 tex of the fetal gonad and that the testis 

 is derived from the embryonic medulla. 

 Ponse (1949), in an extensive and careful 

 discussion of the embryology of the gonad, 

 concludes that a cortex and medulla cannot 

 be distinguished in the developing gonad of 

 the fish. D'Ancona (1950) shares this opin- 

 ion. Studies of the brook lamprey, Ento- 

 sphemis irilderi (Okkelberg, 1914, 1921). 

 and of the closely related Petromyzon (Lu- 

 bosch, 1903) reveal that male and female 

 gonads are at first indistinguishable mor- 

 phologically (undifferentiated gonad stage). 

 Later (bisexual stage) a distinction can be 

 made on the basis of the relative numbers 

 of spermatocytes and oocytes; both kinds 

 of germ cells appear in both sexes. Pro- 

 longed juvenile bisexuality resulting from 

 arrest of sexual develoiiment at the end of 

 the second stage is probably the basis for 

 much of the hermaphroditism in fish. In 

 a third phase (stage of sexual differentia- 

 tion ) , the heterosexual germ cells disappear 

 from the lamprey gonad or persist in rudi- 

 mentary form, and the sex of the lam})rey 

 finally becomes apparent. D'Ancona (1950) 

 summarizes evidence for similar develop- 

 ment of the gonad in several other jilagio- 

 stomes. 



Much, or even all, of the development 

 of tlie o\'ary and testis in both lower orders 

 of fisli and teleosteans may occur after 

 hatching or birth. In Gobius the gonads 

 are said not even to appear until 15 days 

 after birth ( MacLeod, 1881 ) . 



Knowledge of the development of the 

 reproductive system of the teleosts, as of 

 the cyclostomes, is fragmentary. D'Ancona 

 (1950) divides the teleosteans which have 

 been studied into two groups. In the first, 

 the gonads gradually differentiate from an 



indifferent to a bisexual stage and then into 

 ovaries or testes. The viviparous teleost 

 Cymatogaster is an example (Eigenmann, 

 1897). The gonadal anlage in the 8-mm. 

 lar^'a is a simple fold of peritoneum on 

 either side of the mesentery. Each fold ac- 

 quires a core of germ cells and stromal cells, 

 and the two folds, or germinal ridges, fuse 

 posteriorly. In 15- to 17-mm. larvae the 

 ovaries can be distinguished because they 

 are shorter than the testes and have a lon- 

 gitudinal groove which will later invaginate 

 further to form the central cavity of the 

 ovary. The testes develop internal lobules 

 and a central, branched collecting tubule. 

 At 22 mm. the ovaries have become tubular; 

 the walls are thick superiorly and medially 

 where "oviferous folds" will develop, and 

 ai'e thin inferiorly and laterally. Later the 

 two ovarian cavities join posteriorly. The 

 oviducts develop much as in higher verte- 

 brates: a plate is grooved into two parallel 

 ridges, and the latter bridge over the in- 

 tervening space to form a tube which is 

 then extended caudally. Ovaries and ovi- 

 ducts are continuous. The vas deferens, 

 which is not homologous with the oviduct, 

 is lined with stromal cells from the testis. 



Jolmston ( 1951 ) gives a similar descrip- 

 tion for another acanthopterygian, Mi- 

 cropterus, the black bass. He adds that the 

 testis also becomes tubular; the cavity 

 proper, or testocoel, functions as a primary 

 collecting duct. Branches from the testocoel 

 complete the collecting system. A similar 

 development of the gonads and gonoducts 

 appears to occur in several other genera.^^ 



D'Ancona's (1950) second group of tele- 

 ost fish includes representatives of the 

 Sparulae and Serranidae (Dantchakoff, 

 1936; Kinoshita, 1936; Lavenda, 1949; 

 D'Ancona, 1950). Although these are also 

 acanthopterygian families (see above) the 

 embryonic gonads are persistently ovotes- 

 tcs. Both ovarian lamellae and testicular 

 lobules may protrude into the central cavity 

 of the same gonad. In Spams longispinis 



"Anna (D'Ancona, 1955). eel (Grassi, 1919), 

 the trout (Mrsic, 1923, 1930; Asliby, 1952), Xiphu- 

 phorus (Essenberg 1923; Regnier, 1938; Vallowe, 

 1957), pipe fish, sea horse, and goby (MacLeod, 

 1881), goldfish (Stromsten, 1931), guppy (Dildine, 

 1936), and Cottns (Hann, 1927). 



