138 Embryology and Growth of Fishes 



growing mesoderm. In the next stage the closure of the blas- 

 topore is rapidly becoming completed; in front of it stretches 

 the widened and elongated form of the embryo. The yolk-plug 

 is next replaced by periblast, the dorsal lip by the tail-mass, or 

 more accurately the dorsal section of the germ-rim ; the ccelen- 

 teron under the dorsal lip has here disappeared, on account of 

 the close approximation of the embryo to the periblast; its last 

 remnant, the Kupffer's vesicle, is shortly to disappear. The 

 germ-layers become confluent, but, unlike the sturgeon, the 

 flattening of the dorsal germ-ring does not permit the forma- 

 tion of a neurenteric canal. 



The process of the development of the germ-layers in 

 Teleosts appears as an abbreviated one, although in many of 

 its details it is but imperfectly known. In the development of 

 the medullary groove, as an example, the following peculiarities 

 exist: the medullary region is but an insunken mass of cells 

 without a trace of the groove-like surface indentation. It is only 

 later, when becoming separate from the ectoderm, that it ac- 

 quires its rounded character; its cellular elements then group 

 themselves symmetrically with reference to a sagittal plane, 

 where later, by their dissociation, the canal of the spinal cord 

 is formed. The growth of the entoderm is another instance of 

 specialized development. In an early stage 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 noto- 

 chord. In a following stage of development 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 approxi- 

 mation of the entoderm-cells in the median ventral line, the 

 condition is reached where the completed gut-cavity exists. 



The formation of the mesoderm in Teleosts is not definitely 

 understood. It is usually said to arise as a process of ' de- 

 lamination,' 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 the Teleost does 

 not differ from that in other groups; an evagination of the 

 entoderm coming in contact with an invaginated tract of 



