The gonad ridge increases in size both by division of the 

 contained cells and also by migration of cells from the med- 

 ial border of the mesonephros. These migrating cells move 

 down through the supporting mesentery and enter the 

 gonad. The migrating cells form segmental masses which 

 fill the interior of the indifferent gonad, which now has a 

 cortex of germ cells embedded in epithelial cells and is en- 

 closed in a thin epitheliuin. At the anterior and posterior 

 ends of the gonad, the progonal and epigonal portions, sex 

 cells are lacking. 



Near the center of the gonad, small openings or rifts ap- 

 pear between the cortical mass and the medullary se.x cords. 

 These soon widen and join to form the genital cavity or 

 gonocoel. The simple gonocoel is essentially retained in the 

 development of the ovary, while in the testis it is replaced 

 by tubular channels arising within the mass of sex-cord or 

 medullary cells. 



In the development of the ovary, the amount of meso- 

 nephric tissue entering the gonad is small. As growth pro- 

 ceeds, this medullary tissue forms relatively less of the organ 

 although it does increase in absolute amount. The cortex 

 develops from the single layer of germ cells of the indifferent 

 gonad to one with several layers; large oocytes form clumps, 

 or egg nests, within this cortex. The ovocoel (gonocoel of 

 ovary) is divided into many compartments enclosed by 

 medullary cells. 



The testis may develop directly or indirectly. In direct 

 development the germ cells move out of, or are taken out 

 of, the thin cortex by the sex-cord strands and as a result lie 

 scattered throughout the interior of the gonad among the 

 medullary cells. After the sex cells form clumps, a rift ap- 

 pears in each clump, forming the cavity of the ampulla, 

 which is enclosed by the sex-cord cells. The ampulla elon- 

 gates to form a tubule which is connected with the rete testis 

 formed in the medullary mass along the hylus of the gonad, 

 next to the supporting mesentery. The efferent tubules arise 

 from strands of mesonephric cells in the mesentery connect- 

 ing the rete with the marginal canal of the kidney. 



Indirect development occurs in many races of frogs where 

 it may be the only method or only a rare aberration. In this 

 type of development, a gonad appears which at first tends 

 toward cortical development; it produces lobules of cells 

 comparable to the egg nests of the ovary. This gonad also 

 has medullary tissue, enclosing the gonocoel, and this tissue 

 contains sex cells. In form it is elongated like the ovary but 

 is somewhat more irregularly shaped. The female tissues are 

 better developed anteriorly and essentially undeveloped 

 posteriorly. Later the central sex-cord area, enclosing the 

 now subdivided gonocoel, develops along with its enclosed 

 germ cells. The cortical material aborts as this process goes 

 on and a definitive testis results. 



Hermaphroditism (Figure 10-39) is occasional in adults 

 as a result of failure of the sex-directing mechanism to con- 

 vert the indifferent gonad to one sex or the other. In these, 

 the anterior part of the gonad is female, while the more pos- 



terior parts are male. This agrees with the anterior-posterior 

 sexual gradient observed in bufonids. 



Choanates 



Actinistian In Lalimeria the right testis is two or three 

 times larger than the left but both produce sperm. The right 

 testis extends through the middle quarter of the body cavity 

 ventrolateral to the swim bladder. The epididymis is small 

 and leads to the vas deferens, which enters the cloaca sep- 

 arate from the urinary sinus. 



The ovary of the female has a central cavity. The Miil- 

 lerian ducts have not been described. In both sexes, masses 

 of hemopoietic tissue are associated with the reproductive 

 tract as well as with the viscera in general. In this respect 

 the coelacanth resembles the other fishes. 



Dipnoan 



THE MALE The tcstis of Lejndosiren is an elongate mass, 

 round in cross section and enclosed in fatty tissues. It lies 

 somewhat lateral to and below the kidney; its anterior half 

 is in contact with the body wall, while its posterior half is 

 suspended by a mesorchium. 



Internally it is formed from a large number of radially 

 arranged ampullae opening into a central net of canals ly- 

 ing along the dorsomedial margin next to the supporting 

 septum. The central net extends back as an elongate band 

 of tubules closely bound to the kidney tissue. From this 

 band, efferent tubules extend into several of the posterior 

 segments of the kidney; these connect with the glomeruli of 

 the nephric tubules at the time of breeding. The cranial end 

 of the Miillerian duct, with an ostium, is present in the 

 young male but is lacking in the adult. 



The testis of Pwtopterus is proportionally larger than that 

 oi Lepidostren (Figure 10-41). It extends the entire length of 

 the body cavity as a thick band, closely and broadly bound 

 to the kidney above and hanging free in the body cavity 

 below. Anteriorly it is bound to the body wall and appears 

 to be largely lymphoid in nature. The right gonad is at- 

 tached anteriorly to the liver. The left gonad extends for- 

 ward to the region of the duct of Cuvier. Posteriorly it is 

 rounded off but has a short tubular extension. The seminif- 

 erous tubules are much longer than the ampullae of Lepi- 

 dostren. The tubules enter a central canal which extends 

 along the dorsomedial margin of the organ and beyond the 

 posterior end of the testis. The canalicular or rete extensions 

 of right and left testis unite to form a median tube, which 

 ends blindly just anterior to the urinary papilla (Figure 

 10-41 B). From this terminal pocket, efferent ducts enter 

 the kidney to right and left and connect with a large num- 

 ber of renal capsules of the last two segments. The cranial 

 and caudal ends of the Miillerian ducts remain; the rem- 

 nants fuse at the midline but do not enter the urinary sinus. 



The testis of Neuceratodus is long and thick, tapered ante- 



THE REPRODUCTIVE SYSTEM • 327 



