16 EEPRODUCTION AND DEVELOPMENT OF TELEOSTEAN FISHES 



microscope when the blastoderm has almost entirely enveloped the 

 yolk. In the ova fertilized on April 12th, at 1 p.m., and kept 

 under the conditions already detailed, segmentation was completed 

 on the first day, and on the second the extension of the blastoderm 

 over the yolk took place. On the third day the differentiation of 

 the optic vesicles and of the mesoblastic somites commenced. On 

 the fourth day the lens and auditory vesicles were formed, and the 

 first development of pigment appeared as black dots on the sides of 

 the embryo. Fig. 7 shows the condition on the fifth day : the 

 intestinal tube is formed, the notochord is present and already 

 shows the multicolumnar arrangement of its vacuoles, and the 

 cavity of the heart has appeai^ed as a simple slit in the mesoblast 

 below the neck. The black chromatophores are but little further 

 developed. A number of the larvse hatched out on Api-il 19th at 

 the end of the seventh and commencement of the eighth day after 

 fertilization. The structure of the larva is shown in fig. 8. It is 

 3"8 mm. in length. There are now yellow chromatophores as well 

 as black, and both are dendritic. They are present in the median 

 fin-fold, on the sides of the body, and on the surface of the yolk, 

 where they are situated at the surface of the periblast. The mouth 

 is not open, the nasal pit is seen at the anterior end of the head, 

 the heart is more developed and contracts regularly, but there are 

 no red corpuscles. The anus is open and situated immediately 

 behind the yolk. There is a large cavity (the venous sinus) in 

 front of the yolk, between it and the anterior abdominal wall. The 

 notochord is not altered. Besides the median fin-fold there are 

 rudiments of the pectoral fins in the form of a semicircular fold of 

 membrane on each side behind the auditory vesicle. The cavity of 

 the intestine is plainly visible. The larva four days after hatching 

 (fig. 9) is considerably more developed. The mouth is not opened, 

 but its cavity is large and only separated from the exterior by a 

 thin membrane, and three or four gill-slits are open leading from 

 the pharynx to the exterior. The yolk is almost absorbed, and the 

 abdominal region therefore does not protude so much. The venous 

 sinus is still large and its communication with the posterior end of 

 the heart is plainly seen. Red corpuscles are still absent, but 

 white corpuscles or leucocytes are to be seen moving in the venous 

 sinus and passing into the heart. In front of the remnant of the 

 yolk is seen the liver, as a bulbous follicular outgrowth from the 

 wall of the intestine. The urinary bladder is visible behind the 

 rectum. The head is much shortened, so that the auditory vesicle 

 is much nearer to the eye. The pectoral fin is considerably 

 developed, but no fin rays are present in it. The pigment is much 

 more abundant, and has a definite arrangement. The yellow 



