DEVELOPMENT OF THE DOG-FISH. 43 



canal. By separation of these two layers of mesoblast from one 

 another, a cavity is found, which afterwards becomes the general- 

 body-cavity or pleuro-peritoneal cavity. This splitting of meso- 

 blast does not extend quite up to the walls of the medullary 

 canal, hence there is left along either side of the canal, between it 

 and the line along which the splitting begins, a tract or plate of 

 uncleft mesoblat, which receives the name of vertebral plate. The 

 moreexternal mesoblast, viz.jth^ whic*i <?nTitsis called lateral plate. 

 In this vertebral tract the protovertebrse appear. These are 

 merely the mesoblast cells of the vertebral plates cut up into 

 square masses. They do not correspond to the true vertebrae, for 

 not only are the true vertebrae produced by them, but also some 

 of the muscles and spinal nerves. It must be carefully noted, 

 that in that part of the embryo which forms the head, the meso- 

 blast is never cut up into protovertebrse, and never undergoes 

 cleavage. The mouth first appears as a pit in the skin, which 

 deepens until it abuts against anterior end of alimentary canal. 

 Communication is made between the two by a solution of the 

 intervening walls. The cloaca is similarly formed. Meanwhile, 

 other parts of embryo are being developed, which may be 

 described systematically. 



I.-^DEVELOPMENT OF HEART AND BLOOD-VESSELS. 



Heart is formed as an elongated tube (contractile) out of a 

 mass of mesoblast lying underneath the anterior part of the 

 alimentary canal. To provide channels for the fluid thus pressed 

 by the contraction of the heart, a system of tubes makes its 

 appearance in the mesoblast throughout the embryo. 



II. — DEVELOPMENT OF SPINAL CORD AND BRAIN. 



We have seen how primitive cord was formed. It is at first a 

 canal, the cavity of which is of considerable width. By a multi- 

 plication of epiblastic cells which line it, the cavity is much 

 narrowed. Thus the anterior and posterior columns and roots of 

 the spinal nerves are formed, as well as the commissures. The 

 cavity of the canal is now a mere slit, which is soon divided by a 



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