172 THE EYES OF THE BLIND VERTEBRATES OF NORTH AMERICA. 



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 nine hours, when the embryo encircled about one-third of the yolk. Sixteen hours 

 later the blastopore was closing. There is a regular change in the position of the 

 embryo with development. The blastoderm is formed at the side of the yolk. When 

 the gastrula covers half the yolk the egg has rotated so that the gastrula covers more 

 of the lower than of the upper surface of the yolk. Still later, some hours before the 

 closing of the blastopore the latter structure lies to one side of the yolk-sphere, which 

 always occupies the upper pole of the egg; the embryo extends from this region 

 obliquely over the yolk. After the formation of the tail the embryo is always found 

 coiled about the upper half of the yolk. 



The embryo hatches when it is about 5 millimetres long (Plate XII, Fig. 1). 

 The metamorphosis of the larva into the definitive fish is completed before it leaves 

 the gill-cavity of the mother. The longest individuals I have secured from the 

 gill-cavity measure about 10 millimetres. 



VI. THE DEVELOPMENT OF THE EYE. 



i. Earliest Stages to a Length of Three Millimetres. — The development of the 

 eye has been followed in several series of living embryos and in sections of these 

 embryos. I shall describe the earlier stages of the eye as they were observed in the 

 series obtained on May 4, 1901.* Where advisable other series will be described also. 

 The first indications of the eye are seen in living specimens when the embryo is about 

 1.5 millimetres long, at about the time of the formation of the first protovertebra. 

 This size was reached in the present series in two and a half to three days from 



* 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. 

 May 4, 9.00 p.m. Gastrula covers half of the yolk. 

 " 5, 6.00 a.m. Blastopore 1.2 mm. in diameter; embryo surrounds about one third of the yolk. 

 " 5, 2.30 p.m. Embryo 1.6 mm. long and with four protovertebrae. 

 i' 5, 6.00 p.m. Embryo 1.76 mm. long and with six protovertebrae. 



" 5,10.00 p.m. Blastopore closing, embryo 1.92 mm. long and with ten protovertebrae; eyes well formed. 

 " 5, 11.30 p.m. Some embryos 2.4 mm. long. 



' 6, 6.00 a.m. Twelve or thirteen protovertebrae; neural cavity formed. 

 ' 6, 8.00 a.m. Embryo 2.4 mm. long. 

 ' 6, 11.00 a.m. No marked change. 



;< 6, 6.00 p.m. Tail beginning to bud out; embryo, 3 mm. long, encircles half the'yolk; 17 protovertebra 

 present. 



