550 PISCES FISHES. 



The mesoblast arises as two lateral plates, one on each 

 side of the medullary groove. The plates seem to arise as 

 a pair of solid outgrowths from the wall of the gut. They 

 are afterwards divided into segments. Between the meso- 

 blast plates, along the mid-dorsal line of the gut, the 

 notochord is established (Fig. 290, n.). 



Besides the internal establishment and differentiation of 

 layers, there are two important processes, (a) the growth 

 of the blastoderm around the yolk, (b) the folding off of 

 the embryo from the yolk. The result of the two processes 

 is that the yolk is enclosed in a yolk-sac, with which the 

 embryo is finally connected only by a thin stalk the 

 umbilical cord. 



The history of the yolk is briefly as follows : It is accumulated 

 by the ovum from neighbouring cells, and from the vascular fluid ; it 

 is partly prepared for absorption by the merocytes or yolk-nuclei ; it is 

 at first absorbed by the blood vessels of the yolk-sac ; at a later stage, 

 absorption by blood vessels becomes less and less important, and the 

 yolk passes inside the -embryo and into the gut, where it is digested. 

 Then the yolk-sac, empty of all but merocytes, degenerates, shrivels, 

 and disappears. 



Second type of FISHES. The Haddock (Gadus aglefinus) 

 A type of Teleosteans with closed swim-bladder 

 (Physoclysti). 



Form and external features. The elongated wedge-like 

 form is well adapted for rapid swimming. The lower jaw 

 bears a short barbule, long in the cod (G. morrhua)^ 

 absent in the adult whiting (G. merlangus). The nostrils, 

 situated near the end of the snout, have double apertures. 

 The eyes are lidless, but covered with transparent skin. 

 Over the gill chamber and the four gills lies the operculum, 

 supported by several bones. Distinct from one another, 

 but closely adjacent, are the anal, genital, and urinary 

 apertures, named in order from before backwards. Along 

 the sides of the body runs the dark lateral line containing 

 sensory cells. There are three dorsal and two anal fins, 

 and an apparently symmetrical tail fin. 



Skin. The small scales are developed in the dermis, 

 and consist of flexible structureless bone (vitrodentin). 



