155 



sotne of the eggs were in the eight-cell stage or in the sixteen- 

 cell stage. Twelve hours after fertilisation the greater part of 

 the eggs presented a blastoderm with numerous small cells (tempe- 

 rature of the water 15,9° C). Eight hours later the first signs 

 of gastrulation appeared (temperature 16,5° C). Twenty-eight 

 hours after fertilisation the yolk was about half-co vered by the 

 blastoderm and the embryonic shield was clearly outlined. 



Forty-eight hours after fertilisation the blastopore was closed, 

 but Kupffer's vesicle was still visible; the first traces of pigmen- 

 tation appeared behind the eyes (Plate VII, fig. 3). During this 

 time the temperature of the water had ranged from 16 to 17,5° C. 

 The greater part of the eggs were now lying at the bottom of 

 the vessel. The same occurred in other series, and seems to be 

 the natural condition, the eggs presenting an entirely healthy 

 appearance. During the following hours the pigraentation increased, 

 but only slightly, and always in a very peculiar and characteristic 

 distribution. There is no tracé of yellow pigment, and the black 

 pigment at first forms a line at both sides of the fourth ventricle 

 and of the spinal cord. Then it forms a dot on the snout, two 

 conspicuous dark spots behind the eyes, two smaller ones some 

 way behind the auditory vesicles, a black spot near the vent on 

 the caudal side of the rectum and a pigmentbar half-way down 

 the tail. A few pigmentcells are scattered between, the oil-globule 

 is covered with some five or six pigmentcells (Plate VII, fig. 4). 

 But for these spots the embryo is entirely translucent. At about 

 120 hours after fertilization (temperature of the water ranging 

 from 15 to 17° C.) the embryos were hatched. 



In another series the average temperature of the water was a 

 shade higher, at least in the beginning, and the embryos were 

 hatched at about 110 hours after fertilization. The general result 

 of these experiments gave for the development from fertilization 

 until hatching about 1780 »Gradstunden". 



Immediately after hatching the buoyancy of the yolk-sac causes 

 the little larvae to float helplessly in the water, the yolk-sac 

 uppermost, but very soon they are able to keep themselves in the 



