94 BLIND VERTEBRATES AND THEDZ EYES. 



When the belligerents meet, one above the other, the snapping and punching is 

 of a different order. While jerking through the water, just after a round, if one 

 of the belligerents touches one of the neutrals in the aquarium, it frequently gives 

 it a punch, but does not follow it up, and the unoffending fellow makes haste to 

 get out of the road, the smaller ones most quickly. If, after an interval of a few 

 seconds, a belligerent meets a neutral, they quietly pass each other without paying 

 any further attention ; whereas if the two belligerents meet again, there is an im- 

 mediate response. Whether they recognize each other by touch or by their mutual 

 excitability, I do not know. In another aquarium I saw one belligerent capture 

 the other by the pectorals. After holding on for a short time it let go, and all dif- 

 ferences were forgotten. The thrust is delivered by a single vigorous flip of the 

 tail. These fights were frequently noticed, and, as far as determined, always 

 occurred between males. 



The absence of secondary sexual differences in the cave fishes is a forcible 

 argument in favor of sexual selection as the factor producing high coloration in 

 the males. The absence of secondary sexual differences in caves opposes the idea 

 of Geddes and Thomson, that the differences are the external expression of maleness 

 and femaleness. 



THE EGG AND GENERAL DEVELOPMENT OF AMBLYOPSIS. 



The eggs are large, measuring 2.3 mm. in diameter. The yolk is translucent, 

 of various tints of amber. The yolk measures 2 mm. in diameter and contains a 

 large protruding oil-sphere 1 to 1.2 mm. in diameter. When the egg is deposited, 

 the yolk is flabby and composed of yolk-spheres of various sizes loosely put 

 together. After the egg has been in water for some time, the yolk forms a tense 

 rounded mass. The egg is heavier than water. The oil-sphere lies uppermost 

 in the egg, and the germinal disk forms at the side of the egg. Attempts at artificial 

 fertilization have not been successful beyond obtaining well-developed germinal disks. 



The rate of development will probably be found to vary considerably with the 

 temperature of the water. In a series of eggs in which the gastrula covered half 

 the yolk when observations began, the blastopore was reduced to the size of the 

 oil-sphere in 9 hours, when the embryo encircled about a third of the yolk; 16 

 hours later the blastopore was closing. The rate of development of the series of 

 eggs taken in May was as follows, the mother containing the eggs having been 

 kept in a small aquarium without change of water and at the temperature of an 

 ordinary living room. The temperature of the water in the cave is 12 C, that 

 in the room was 22 C. 



On May 4, 9 p. m., the gastrula covered approximately half the yolk. It lies eccentric, neither 

 below nor at the side, the germ being evidently heavier, the oil-sphere at the top. 



May 5, 6 a. m., the embryo surrounds about a third of the egg, the blastopore is about as wide 

 as the oil-sphere, 1.2mm., and the latter seems to fully fill it. At2. 30p.m. the embryo is 1. 6mm. long 

 and has 4 protovertebras. At 6 p. m. the blastopore has narrowed considerably and invariably lies 

 at one side of the oil-sphere, the embryo lying oblique to the vertical axis of the egg. This eccentric 

 position becomes more and more evident as the blastopore closes toward 10 p. m. The embryo is 1.76 

 mm. long, with 6 protovertebrae. At 10 p. m. the eyes and brain are shaped like the ace of spades, 

 the eye lobes evidently not yet narrowly separated from the brain by a narrow stalk, the blastopore 

 closing, the embryo 1.92 mm. long, and with 10 protovertebrae. On May 6, at 6 p. m., the embryo 

 lies horizontal around the margin of the yolk ; the cavity of the central nervous system has appeared ; 

 a large piece has been eaten out of the yolk; the lens is just beginning to develop. There are 12 

 or 13 proto vertebras. At 8 a.m. the embryo is 2.4 mm. long; at n a. m. no marked change is seen ; 

 at 6 p. m. tail is beginning to bud out; embryo, 3 mm. long, encircles half the yolk; 17 proto- 

 vertebrae present. 



