306 



THE ELASMOBRANCH FISHES 



Fig. 266. Ovidiical valve, SquaJus sucMii. 

 (F. Hurni, orig.) 

 od., oviduct; ut., uterus; vl., valve. 



The young, thus protected by the shell and supplied with an abundance of 

 food yolk, undergo a period of development outside of the body. This period 

 varies greatly depending largely upon the temperature of the surrounding 



water. Under favorable conditions the 

 eggs may hatch in six or seven months, 

 but the period is more likely to ap- 

 proximate nine months. At the end of 

 this time hatching is accomplished bj' 

 the perforation of the upper end 

 (Scyllium) , or the lower eiid (Baja) 

 of the shell. In Heterodontus the two 

 layers at the large or perforate end 

 separate making a large aperture 

 through which the young emerges. 

 In many of the Elasmobranchs a temporary shell is formed which serves 

 the young fish only through its early development. From this temporary shell 

 the embryos emerge and undergo more or 

 less of their development in the oviduct of 

 the mother (Acanthias, Mustelus, Rhino- 

 hat is). 



A temporary shell is very often a struc- 

 ture of exquisite beauty. In Squalus 

 SKcklii it consists of a long thin-walled 

 tube of a clear amber color, each shell con- 

 taining from four to six eggs. The eggs 

 undergo their early development incased 

 and protected until the time when the ex- 

 ternal gills begin to be absorbed. The shell 

 then ruptures and the young embryos 

 take up their development in the uterus 

 of the mother. In Rhmohatis a similar 

 shell is found and after it has been dis- 

 carded by the embryos it may be found 

 rolled up in the uterus. 



UTERUS 



Fig. 267. A. Section of uterine lining 

 to show villi and blood supply, Squahis. 

 B. Transverse section through a single 

 villus, Acanthias. (From Brinkmann.) 



cp., capillaries; t.a., terminal artery; 

 vi., villus. 



We have said that the uterus is much more 

 highly developed in those sharks which 

 give birth to living young; for in these it serves as a place in which a consider- 

 able part of the development is undergone. Such a uterus is that of Squalus 

 sucklii {ut., fig. 253) in which it may be a greatly enlarged sac with well de- 

 fined boundaries. Anteriorly the uterus of Squalus is separated from the for- 

 ward part of the oviduct by a well defined oviducal valve (fig. 266) which is a 

 wavy constriction with a very narrow lumen effectively closing the uterus to 

 the upper part of the oviduct. 



