MARINE TELEOSTEAN DEVELOPMENT. 93 



three weeks after hatching the branchial system is wholly 

 converted into cartilage. In general the arches have a meso- 

 blastic core and a hypoblastic epithelial covering. 



Skull. 



At first the brain is covered by a thin epiblastic layer com- 

 posed of the flattened corneous stratum, and the thick sensory 

 remnant beneath. A sub-epidermal enlargement often occurs 

 beneath this in larvae in confinement, giving a peculiar outline 

 to the animal. Then the mesoblast is aggregated between the 

 eye and the neurochord, and, passing upward as a thin 

 membrane, invests the brain. Ultimately the three brain-mem- 

 branes are developed from it, and pigment is also formed on its 

 inner surface. At the time of hatching the base of the brain 

 is strengthened by two masses (parachordals), and in front 

 of these are two cylinders (trabecule), which unite posteriorly. 

 Within a week or 10 days after hatching these are converted 

 into cartilage-cells. About the middle of the third week 

 various other cartilages of the skull are formed, e.g. the optic 

 and auditory, but the translucent opercular plates on the 

 surface of the more exposed skeletogenous elements of the face 

 occur later (post-larval stage). While cartilage thus early 

 develops in the skull, that of the axial skeleton is later. 



Alimentary canal. 



In its earliest condition the alimentary tract consists merely 

 of a thickened layer of hypoblast, intervening between the 

 neurochord above and the periblastic covering of the yolk 

 below. When little more than one-third of the yolk is covered 

 by the blastoderm the hypoblastic cells assume a columnar 

 character, arching over a lumen — beneath which is a floor of 

 periblast. This is the first indication of a gut — the continuity 

 of which with the neurenteric canal and medullary groove has 

 been described. From the arched roof the notochord is diffe- 

 rentiated. The lumen extends only a short distance forward, 

 and is lost in hypoblastic cells which reach as far as the cardiac 

 region, where they spread out and form a delicate limiting mem- 

 brane below the head. The ventral wall of the canal is formed 



