TELEOSTEI. 75 



late a division into splanchnic and somatic layers, between 

 which is placed the primitive body cavity. The dorsal part of 

 the plates becomes transversely segmented in the region of the 

 trunk ; and thus gives rise to the mesoblastic somites, from 

 which the muscle plates and the perichordal parts of the 

 vertebral column are developed. The ventral or outer part 

 remains unsegmented. The cavity of the ventral section 

 becomes the permanent body cavity. It is continued forward 

 into the head (Oellacher), and part of it becomes separated off 

 from the remainder as the pericardial cavity. 



The hypoblast forms a continuous layer below the mesoblast, 

 and, in harmony with the generally confined character of the 

 development of the organs in Teleostei, there is no space left 

 between it and the yolk to represent the primitive alimentary 

 cavity. The details of the formation of the true alimentary tube 

 have not been made out ; it is not however formed by a folding 

 in of the lateral parts of the hypoblast, but arises as a solid or 

 nearly solid cord in the a'xial line, between the notochord and 

 the yolk, in which a lumen is gradually established. 



In the just hatched larva of an undetermined fresh-water fish with a very 

 small yolk-sack I found that the yolk extended along the ventral side of the 

 embryo from almost the mouth to the end of the gut. The gut had, except 

 in the hinder part, the form of a solid cord resting in a concavity of the yolk. 

 In the hinder part of the gut a lumen was present, and below this part the 

 amount of yolk was small and the yolk nuclei numerous. Near the limit 

 of its posterior extension the yolk broke up into a mass of cells, and the 

 gut ended behind by falling into this mass. These incomplete observations 

 appear to shew that the solid gut owes its origin in a large measure to nuclei 

 derived from the yolk. 



When the yolk has become completely enveloped a postanal 

 section of gut undoubtedly becomes formed ; and although, 

 owing to the solid condition of the central nervous system, a 

 communication between the neural and alimentary canals 

 cannot at first take place, yet the terminal vesicle of the post- 

 anal gut of Elasmobranchii is represented by a large vesicle, 

 originally discovered by Kupffer (No. 68), which can easily be 

 seen in the embryos of most Teleostei, but the relations of which 

 have not been satisfactorily worked out (vide fig. 34, hyv). As 

 the tail end of the embryo becomes separated off from the yolk 

 the postanal vesicle atrophies. 



