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 1/7 in Fig. 282. It is only later, 
when becoming separate from the ectoderm, EC, that it 
acquires its rounded character (Fig. 279), 17; 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), JZ. 
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 asa process 
of ‘delamination,’ ze. 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 
