113 



The Microscope. 



downward, then reverse it, and if the great bulk of spawn be seen to 

 shift and drop as from one end of the fish's belly to the other, the 

 eggs are loose and the fish is ready to part with its ova" (Fig. 1). 



(From Francis.) 



The method usually advised for holding the fish and expressing the 

 ova is shown in Fig. 2. A still better method introduced by Mr. 

 Glover is to hold the head of the fish with the left hand, seizing the 

 tail in the right hand just behind the vent, so as to compress the 

 back between the fingers and palm of the hand. The fish is then 

 slightly bent, and the side of the thumb rubbed against its belly, 

 just above the vent, which will cause the extrusion of the eggs. This 

 should be done with the fish partially submerged in water. The 

 sperm of the male is then obtained in the same manner, and in the 

 same dish; and the eggs and the sperm gently mixed together by 

 means of a feather. The eggs should now remain for about two 

 hours undisturbed, and then transferred to the hatching trough or 

 grille by a horn spoon and a feather. This hatching box, according 

 to Exner, must have the floor covered with pebbles, while 1. 5 c. m. 

 above this is arranged a layer of glass rods, some 2 to 4 m. m. fi'om 

 each other. Upon these rods the eggs are laid. The box must then 

 be provided with running water, the flow being drop by drop or 

 swifter. Dead eggs may be recognized by their opacity, and should 

 be removed each day. The eggs should be examined eveiy twelve 

 hours. 



Mammalian embryos may be obtained, in the case of sheep, &c., 

 at slaughter houses ; for those of small animals, as rabbits, mice, 

 &c., the female must be killed at a varying period after fecundation 



