82 GENERAL SKETCH OF 



termination of the notochord posteriorly. Usually the terminal 

 part is attenuated, but occasionally it is slightly enlarged. 



Eyes. (Optic nerves and vesicles.) 



One of the earliest features in the embryo is the great 

 development of the cephalic region, which is chiefly due to the 

 protrusion of two rounded lateral masses (optic vesicles) from 

 the sides of the narrow fore-brain. This cellular proliferation 

 later shews a median slit, the only approach to the hollow 

 brain-vesicle in the primitive forms. This slit passes along the 

 stalk into the fore -brain ; and subsequently it is seen that the 

 optic nerves are at a different level. 



Very soon the vesicles become cup-shaped by the pressure 

 of an almond-shaped mass of thickened epiblast the future 

 lens, and the thinner margin creeps over the intruding mass of 

 epiblast so as to form a circular lip around it, except at the 

 lower side. The gap thus formed is called the choroid fissure 

 and persists for some time. Mesoblastic cells, which were 

 included as a thin plate between the optic nerve and the brain, 

 have spread over the front as an outer layer (sclerotico-choroidal 

 sheath) and pushed their way through the choroidal fissure into 

 the internal chamber of the eye, and probably break down to 

 form the vitreous humour and other structures. Just before 

 hatching, pigment occurs outside the optic vesicle and in the 

 external investment. Moreover, no less than six layers of the 

 retina are present before extrusion occurs 1 . 



After hatching, the complexity of the eye is greatly in- 

 creased, one of the most marked features being the develop- 

 ment of well-defined rods and cones. The brilliantly opalescent 

 iris not only adds to the remarkable appearance of the trans- 

 parent larva but, with the pigment of the choroid, generally 

 enables it to be discerned in the tanks. 



Auditory organs. 



The ears (otocysts) are formed, soon after the differentiation 

 of the optic vesicles, as solid proliferations of the sensory layer 



1 Those specially interested should refer to Dr Marcus Gunn's account of the 

 development of the Teleostean eye, Ann. Nat. Hist. Sept. 1888, pp. 263268. 



