MARINE TELEOSTKAN DEVELOPMENT. 87 



only appear in the later post-larval condition as shown under 

 the various species. 



ORGANS FORMED FROM THE MESOBLAST. 



Heart and circulatory system. 



The heart in the Teleosteans here described develops at 

 a very early stage before the oesophagus is formed as a 

 cylindrical cellular process of splanchnic mesoblast in front of 

 the pectoral region, that is, between the otocysts and the optic 

 vesicles. 



After the alimentary canal is defined and when about 

 24 muscle plates are marked off, the heart has a vermiform 

 shape and is still solid. A fissure soon appears in the centre, 

 and the heart becomes a simple tube, then muscular twitchings 

 and finally pulsations occur (48 to 60 per minute). The cavity 

 is lined by a single layer of cells, but no fluid is yet present. 



Then the cardiac tube becomes conical, and a pericardial 

 chamber is apparent, also formed from mesoblast. The heart 

 next becomes L-shaped, the arterial end being median, the 

 auricular passing off at right angles. The cardiac lining 

 becomes papillose, and granules may occasionally be seen at 

 the open end of the auricle which is in communication with 

 a space the germinal cavity. This cavity becomes indistinct 

 after the closure of the blastopore but again becomes evident 

 between the periblast and epiblast of the yolk-sac (periblastic 

 blood-sinus of Cunningham). The latter author is of opinion 

 that this cavity is theoretically no longer the segmentation- 

 cavity, since it is on its inner or periblastic side partially 

 lined by mesoblast-cells, viz. chromatophores from the periblast. 

 Morphologically, it is homologous with the vitelline vessels of 

 the salmon, and, similarly, is continuous with the auricle of the 

 heart and shut off from the pericardium by a definite meso- 

 blastic membrane. 



Meanwhile vascular canals are in course of formation, the 

 dorsal aorta being hollowed out of the mesoblastic cells above 

 the gut and under the uotochord, and two lateral venous trunks 



