MARINE TELEOSTEAN DEVELOPMENT. 75 



larval yolk-sac after hatching, viz. in the torsk or tusk'. More- 

 over, black pigment at a somewhat later stage develops in the 

 periblast bordering the yolk. 



We may further note that in demersal eggs with vitelline 

 circulation, the germinal cavity disappears, after closure of the 

 blastopore, partly by the pushing in of mesoblast. 



When the blastopore closes, or a few hours before, a vesicle 

 (Kupffer's) appears on the ventral aspect of the embryo slightly 

 anterior to the tip of the tail (Fig. 24). Occasionally other 

 vesicles occur along the under surface of the embryo, in front of 

 the foregoing. 



At the same time, a canal (neurenteric canal) is seen passing, 

 from the transient homologue of the medullary groove of the 

 dorsum, to the under surface of the embryo. In most other 

 vertebrates, the groove in the dorsum, called the medullary 

 groove, which closes later to form the canal of the spinal cord, is 

 a very early feature in development, whereas in the cod and 

 other Teleosteans the solid mass forming the spinal cord has 

 a fissure formed in its axis only at a later date. In the same 

 way the neurenteric canal and blastopore are much more con- 

 spicuous features of the larval stages of other vertebrates. 



19th April, 5th day. With the closure of the blastopore, 

 the definition of the embryonic fish as an elongated rod pressing 

 into the surface of the yolk, becomes marked. The middle 

 layer or mesoblast has by this time segmented into five or six 

 transverse divisions called muscle-plates or protovertebrae. 

 The primary axis (notochord) can be traced some distance 

 forward, but only as a broad translucent streak. This organ 

 arises from the hypoblast as a median axial rod of cells growing 

 upwards and separating the two rows of mesoblastic lateral 

 protovertebrse. These increase in number rapidly, so that 

 with the extension of the mesoblast forward, the trunk is wholly 

 segmented, and they give the embryo a transversely banded 

 appearance. On either side of the head an invagination of a 

 small area of epiblast gives rise to the lens of the eye, and 

 an expansion of the lateral walls of the embryo on each side 

 forms what is called the alar membrane. 



' W. G. M. Wth Ann. S.B.F. Rep., p. 290. 



