i85 



two cell aspect others showed four divisions, and in a few 

 traces of still further divisions were perceptible. Temperature 

 also has much to do with the rapidity of development. 



The formation of the " segmentation cavity" which appeared 

 in about two hours after fertilization, and the growth of the 

 germinal disc over the yolk need not here be described in 

 detail. It need only be mentioned that in about ten hours the 

 gastrula or expanding mass has spread well over the yolk, its 

 thickened rim being beyond the equatorial region. An hour 

 later the first traces of the embryo were seen when this 

 thickened rim was | over the yolk, and about an hour and a 

 half after this the first traces of the eyes appeared at one end 

 •of the developing embryo, and at the other a small clear spot 

 (Kupffer's vesicle). At this stage the blastopore has closed and 

 the iirst segmentations of the body of the embryo have 

 appeared. Figs. 6 and 7 represent a lateral and ventral view of 

 a slightly later stage in which the segmentations of the body 

 have increased in number, Kupffer's vesicle has disappeared, 

 and spots of pigment are to be seen on the body of the 

 embryo. A characteristic feature of this egg seems to be the 

 temporary appearance of several spots on the yolk between the 

 oil globule and the tail {I'icie fig. 7). These disappear com- 

 pletely soon afterwards. In 49^ hours after fertilization the 

 embryos began to hatch out, and six hours later most had 

 hatched out and were very active. The mean temperature 

 from fertilization to hatching was 65*^ Fahr. Fig. 8 represents 

 one of the fish just after emerging from the egg. It was 2'5 

 millimetres in length and "8 mm. in greatest depth, including 

 the yolk. The front margin of the yolk falls under or slightlv 

 in front of the end of the. snout. The yolk is slightlv oval, 

 being '8 mm. in length and '6 mm. in depth. Immediately 

 behind the yolk is the descending part of the digestion tract. 

 It curves slightly backwards, opening in a small indentation 

 ventrally a little further back, at a distance from the volk about 

 ^ the diameter of the oil globule. The oil globule is about 

 the same size as in the egg, though drawn out slightly in a 

 ■dorso-ventral direction. It is now fixed, and occupies the 

 posterior angle of the yolk sac. The notochord is multi- 

 columnar. The pigment cells, which begin to appear in the 

 embryo at an early stage (about two davs after fertilization) as 

 small spots, yellow (by reflected light) and scattered irregularly 

 along the side of the head and body, being absent from tail 

 yolk and oil globule, have after the hatching process arranged 

 themselves in a more definite manner as follows : Yellow 

 pigment cells with many branchings on the head chiefly 

 behind and on or in front of eyes. Above and below the 

 body over the centre of the yolk there is a branching cell. 



