THE ELASMOBRANCII FISHES 



303 



urogenital. The urogenital sinus, as in Heplnuchus, then emi)ties by means 

 of a urogenital ])ai)illa into the cloaca. 



Leigli-Sharpe (1920-21) has described fully the sii)lions of a number of 

 Elasmobranclis. These are composed of longer or shorter closed sacs which 

 end po.steriorly by siphon tubes. In a type like Acanthias the sac lies under the 

 skin ventral to the base of the pelvic fin, and its tube empties into the proximal 

 part of the clasper tube. In a large specimen of Galiorhinus this siphon sac 

 extended almost to the pectoral girdle. The walls of the sac are muscular and 

 its function appears to be the forcing 

 of the sperm cells through the clasper 

 groove. Glands may line the whole sac 

 as in Lamna or the dorsal side of its 

 wall only (see p. 28, fig. 31) . The func- 

 tion of the clasper gland in the latter 

 condition is not definitely known. 



GENITAL ORGANS OF FEMALE 

 OVARIES 



Fig. 262. Section through shell gland, Scyl- 

 Hum. (From Borcea.) 

 t.gh, secretory cells. 



The ovaries of the adult female usu- 

 ally arise as paired structures, and 

 are bound to the anterodorsal wall of 

 the body cavity by a mesentery, the 



mesovarium (Squalus, fig. 253a). Not infrequently, however, the left ovary 

 atrophies in the adult (Scyllium, Prisfiophorus, Carcharias, Galeus, Mustelus, 

 and Zygaena) . They occupy the anterior part of a mass of tissue which, as the 

 epigonal organ, may extend along the dorsal wall of the body cavity pos- 

 teriorly where it joins the rectal gland. In numerous forms, however, the 

 epigonal organ is wanting as, for example, in Acanthias. The ovary varies 

 greatly, depending upon the stage of maturity of the ova contained. It appears 

 as a sac through the walls of which the ova may be seen varying in size from 

 relatively minute spots to mature ova often from 3 to 5 cm. in diameter (see 

 01'., fig. 252). 



In development an indifferent sex cell divides several times forming 

 oogonia. Each oogonium then undergoes a period of growth to become a pri- 

 mary oocyte. By the first maturation division, this primary oocyte gives rise 

 to the secondary oocyte and first polar cell. The former soon undergoes the 

 second maturation division, thereby forming the ootid and second polar cell. 

 The ootid without further division increases in yolk content to become the 

 ovum. The ovum then breaks through the wall of the ovary. At this stage it 

 contains only one-half the number of chromosomes characteristic of the body 

 cells. If it be fertilized by a spermatozoan, which also bears only one-half the 

 normal number of chromosomes, the number of chromosomes characteristic 

 for the species is restored. 



The place where the egg was located in the ovary now becomes filled up by 

 a corpus luteuni {x, fig. 261). 



