CHAP. Ill GENERATION OF FISHES AND THEIR FECUNDITY 63 



exuded from the sides of the egg-tube and surrounds the yolk. 

 The lower part of the egg-shell is formed before the descent of 

 the ball of yolk, and stops up the passage of the egg-tube so that 

 the yolk rests within it until the remainder of the shell is formed. 



Then the shell, containing 

 the white and the yolk, is 

 separated from the sides of 

 the oviduct and passes 

 down to the end, where it 

 is " laid," or driven out. 

 In skates and the rough 

 dog-fishes there are stiff 

 rods or long tendrils at the 

 four corners of the egg. 

 These are of course formed 

 with the shell : the lower 

 projections are the first 

 part of the shell formed, 

 the upper the last. 



In some dog-fishes, for 

 instance the common spiny 

 dog-fish, no eggs are laid, 

 but the young are born 

 alive. In this case a shell 

 of the usual kind is not 

 formed in the egg-tube, 

 neither is a shell altogether 

 absent. In a female opened 

 before the development of 

 the young has proceeded 

 very far the lower part of 

 each egg-tube is found to 

 contain two or three )'olks 

 enclosed in a single thin 

 transparent horny case, 

 which corresponds to the 

 egg-shell described above. 



OD 



P"iG. 29. — Diagram of a dissection of a female of 

 the Smaller Spotted Dog-fish to .show the 

 ovary and oviducts. O. the right ovary, 

 the only one present in the dog-fish ; CD. 

 the oviduct, containing a ripe egg on its 

 passage towards the exterior ; SG. the 

 gland of the oviduct, or eggshell-gland ; P. 

 the anterior apertures of the two oviducts ; 

 OC. the posterior opening of the oviducts 

 into the "cloaca," behind the opening of 

 the intestine. (After Milnes Marshall.) 



But this egg-shell is not 

 oblong with projections at 

 the corners, but shaped 



