88 



Behind the eye is a small oval cavity containing two specks. This is the primitive ear, 

 the black specks being small particles of carbonate of lime formed within the auditory 

 cavity. Beneath the throat is seen a small tubular structure, which in the living young 

 fish pulsates regularly : this is the heart. From the line where the wall of the yolk sac, 

 or abdomen, joins the body there projects a semicircular membranous fold, which is 

 the beginning of the pectoral fin. No indication of the pelvic fins is yet present. The 

 trunk of the young fish is seen to be divided by a series of curved transverse lines : 

 these are the divisions between the muscular masses of which the trunk muscles are 

 composed. A kind of tube runs down the centre of the body, the contents of which 

 show a recticnlate structure. This tube is the notochord, which contains gelatinous 

 material in a number of cavities divided from one another by thin partitions like the 

 cavities of a sponge. No bone is yet formed, but the vertebras when they develop 

 arise as a series of bony rings formed round the notochord. The chromatophores are 

 now much more abundant than in the embryo before hatching ; there are still only 

 two kinds, the black and yellow, and both are much branched ; the two kinds are 

 everywhere mingled together. The pigment is abundant in the primordial median fin, 

 except at its posterior extremity. 



Fig. 4 is a drawing from another recently-hatched sole, a few hours older than that 

 shown in Fig. 3. It will be observed that this figure shows the left side of the fish, 

 while Fig. 3 shows the right side, and that the two sides correspond in all respects. 

 The newly-hatched sole, or larva of the sole as it may be called, exhibits perfect 

 bilateral symmetry and therein resembles the adults of the greater number of marine 

 fishes. In Fig. 4 the olfactory organ is seen in front of the eye : it is on each side a 

 •simple rounded cavity opening by a small aperture to the exterior. 



It may be explained here, though it is not evident in the figures, that the cavity 

 surrounding the yolk in the larval sole contains primitive blood ; this primitive blood 

 contains minute colourless corpuscles, but no red corpuscles, which are not formed tiU a 

 later stage. The posterior end of the heart is open to this cavity, and the blood is 

 propelled from the cavity along the vessels at the side of the throat into the aorta 

 which runs beneath the notochord, and from the aorta to the cavity again. 



The newly-hatched larva is from 3-55 to 3-75 mm. in length (-142 to -15 inches), or 

 l)etween one-seventh and one-sixth of an inch, and about two and-a-half times as long 

 as the diameter of the ovum. 



Up to this point the stages of development have been described as they are seen in 

 the living egg or newly-hatched fish, and all the figures referred to are from drawings 

 made by the aid of the camera lucida with a low power of the microscope. But, 

 ahhough I have not studied the subject especially in the egg of the sole, it will be con- 

 venient here to give a brief description of the internal processes of development so far 

 as they are known to occxir in the eggs of fishes. 



The germinal ring when examined by means of prepared thin sections is found to 

 consist of three Inyers of cells. The outermost, which is two cells thick, is called the 



