212 



DEVELOPMENT OF FISHES 



The process of the development of the germ layers 

 in Teleosts appears an abbreviated one, although in many 

 of its details it is but imperfectly known. In the develop- 

 ment of the medullary groove, as an example, the follow- 

 ing peculiarities exist : the medullary region at HP (Fig. 

 276) is but an insunken mass of cells without a trace of 

 the groove-like surface indentation of Fig. 261 or 229. 

 Its condition is figured at Mm. Fig. 282. It is only later, 

 when becoming separate from the ectoderm, EC, that it 

 acquires its rounded character (Fig. 279), M \ its cellular 

 elements then group themselves symmetrically with refer- 

 ence to a sagittal plane, where later by their disassocia- 

 tion (.'') the canal of the spinal cord is formed (Fig. 280), M. 

 The growth of the entoderm is another instance of special- 

 ized development. In the section of the embryo of Fig. 

 279, the entoderm exists in the axial region, its thickness 

 tapering away abruptly on either side ; its lower surface 

 is closely apposed to the periblast ; its dorsal thickening 

 will shortly become separate as the notochord. In a fol- 

 lowing stage of development (Fig. 280), the entoderm is 

 seen to arch upward in the median line as a preliminary 

 stage in the formation of the cavity of the gut. Later, 

 by the approximation of the entoderm cells in the median 

 ventral line, the condition of Fig. 281 is reached, where the 

 completed gut cavity exists at G. 



The formation of the mesoderm in Teleosts is not defi- 

 nitely understood. It is usually said to arise as a process 

 of ' delamination,' i.e. detaching itself in a mass from the 

 entoderm. Its origin is, however, looked upon generally 

 as of a specialized and secondary character. 



The mode of formation of the gill slit of a Teleost does 

 not differ from that in other groups ; an evagination of 

 the entoderm, GS (Fig. 282), coming in contact with an 



