L _ left ovary 



outline of cloacol area 



-onus 



- genital pore 

 -urinary pore 



Figure 10-44. Ventral view of female system of the Smelt, Osmerus 

 eperlanus. (After Huxley, 1 883) 



funnel. The oviduct lies lateral to the mesovarium and below 

 the nephric duct. The oviduct enters the nephric duct where 

 the latter turns downward toward the urogenital sinus. The 

 ovary of Amia is like the testis in having a distinct vascular 

 drainage which passes along the middle of the medial 

 aspects and connects anteriorly with the main vascular 

 channels. 



In Lepisosteus the ovary is more elongate than that of 

 Amia. It is sacculate and attached in its middle part by a 

 dorsally tapering, membranous funnel, in part formed from 

 the mesovarium. This funnel leads into the oviduct lying 

 along the body wall below the nephric duct, as in Amia. Pos- 

 teriorly the oviduct enters the nephric duct in the region of its 

 swollen bladder portion. 



The parovarial ovary of Lepisosteus is formed by folding of 

 the genital ridge to meet a sheet extending down from the 

 body wall. The ovary and its oviduct suggest the situation ob- 

 served in teleosts but could be a parallelism. 



Ac'ipenser Acipenser and Polyodon are peculiar in that both 

 sexes have large, funnel-like ducts extending anteriorly, be- 

 tween the mesentery of the gonad and the body wall, almost 

 to the middle of the gonad (Figure 10-45 C,D). The gonads 

 are long bands of tissue, irregularly thickened but thickest in 

 their posterior half They extend from the region of the 

 heart, back nearly to the cloaca. The testis has a distinct 



bundle of tubular tissue, the central net, running along the 

 middle of its lateral aspect. This band passes just below the 

 margin of the coelomic funnel. Anteriorly the testis lies in 

 contact with the body wall, and posteriorly it is suspended 

 by a short mesorchium. The vasa efferentia extend from the 

 central canal band dorsomedially to the margin of the kid- 

 ney. The nephric duct is not modified as a vas deferens. 



The ovary is similar in form. Its eggs rupture from the 

 lateral surface into the body cavity and pass down the coe- 

 lomic funnel. 



Embryological development The development of the gon- 

 ads of actinopterygian fishes is only partly known. The 

 germ cells migrate into the body and come to lie in two bands 

 near the midline mesentery, below or medial to the nephric 

 ducts. These germ cells are surrounded by cells of epithelial 

 and mesenchyme origin, and the whole is enclosed in a thin 

 epithelium. The germ mass now extends down into the body 

 cavity as a genital fold. Development of the swim bladder 

 in the dorsal mesentery tends to move the genital ridges 

 laterally, and thus they are now supported from the sides of 

 that bladder. When the swim bladder contacts the body 

 wall laterally, the genital fold comes to lie below the blad- 

 der, but well away from the median septum. Multiplication 

 of germ cells and stroma (filling-in) cells follows, and an 

 irregular cavity, the gonocoel, forms. This cavity is formed 

 from splits in the gonad tissue. 



From the indifferent gonad, male and female organs 

 develop. Both sexes apparently utilize the internal cavity of 

 the indifferent organ. The female differentiates earlier, with 

 some of the germ cells, the ovogonia, enlarging as ovocytes 

 and the stroma forming a thick outer capsule. The central 

 cavity is now lined by an epithelium and extends back 

 through the epigonal part of the ridge as the oviduct. The 

 oviduct eventually reaches the exterior at the genital pore 

 or enters some other tubular structure. 



In the male the testis is indicated by clumps of germ cells, 

 the spermatogonia, which arrange themselves in the walls of 

 diverticula from the lined central cavity. In this way the 

 tubular or ampullar system is formed. The central canal, or 

 system of canals, extends posteriorly through the epigonal 

 region to form the male duct. 



In some, perhaps most, teleosts the ovocoel opens laterally 

 by a split into the coelom. Thus an ovarian groove is formed. 

 This groove extends posteriorly through the epigonal area 

 as the oviducal groove. This groove becomes lined with 

 epithelium and then closes secondarily to produce the ento- 

 varial or parovarial ovary along with its posterior oviducts. 

 In some fishes a dorsolateral opening remains in the oviduct 

 even in the adult. This slit is functionally closed by contact 

 of its edges with the body wall (Lampanyctus). 



General observations The origin of the oviducts of the 

 primitive types of fishes, such as Po/vpterus, Acipenser, Amia, 

 and Lepisosteus, has not been satisfactorily worked out, but it 



THE REPRODUCTIVE SYSTEM 



331 



