THE HADDOCK. 529 



system. Its sides afterwards arch towards one another, and 

 meet to form a medullary canal (Fig. 255, n.c.). A posterior 

 communication between this dorsal nervous tube above and 

 the ventral alimentary tube persists for some time as the 

 neurenteric canal (Fig. 255, ne.c.). 



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. 255, .). 



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 ceglefinus] 

 -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 terminal 

 mouth bears a short barbule ; this is long in the cod 

 (G. inorrhua\ and absent in the 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 con- 

 taining sensory cells. There are three dorsal and two anal 

 fins, and an apparently symmetrical tail fin. 



34 



