42 DEVELOPMENT OF THE DOG-FISH. 



shape from mutual pressure, so as to resemble pavement 

 epithelium. The deeper cells of the interior pass gradually 

 to the surface and , accumulate there, thus increasing the 

 thickness of the membrane already formed by the more superficial 

 layer of cells, while the Gentral part of the yelk remains filled 

 only with a clear fluid. By this means the yelk is shortly con- 

 verted into a kind of secondary vesicle, the walls of which are 

 composed externally of the original vitelline membrane, and 

 within by the newly formed cellular layer, — the blastodermic or 

 germinal memDrane, as it is called. The blastoderm now splits 

 into two layers, called respectively epiblast and hypoblast. ,A 

 number of cells now appear apparently from out of the substance 

 of the yelk lying beneath the hypoblast, and migrate in between 

 the epiblast and hypoblast, and so form a third layer or mesoblast 

 between the other two. 



A groove then appears on the outer or upper surface of the 

 blastoderm, and named the medullary groove. 



The sides of this groove, which are called medullary folds, 

 rise upwards chiefly from a thickening of the mesoblast, and 

 tend to approach one another. This they ultimately do, first 

 anteriorly, and sometimes afterwards posteriorly. The groove is 

 thus cenverted into a canal — the spinal canal. A fold has mean- 

 while appeared anteriorly in direction at " right angles to the 

 medullary folds. This fold marks the position of the future 

 head, and is hence called the head fold. It assisted in closing in 

 medullary groove in front, while somewhat later a similar fold 

 appeared posteriorly, helping to close in medullary groove behind, 

 called the tail fold. The mesoblastic cells that lie under the 

 floor of medullary groove separate from other mesoblastic cells, 

 and become elongated to form a rod -like structure — the notocord. 

 The hypoblast takes no part whatever in these changes, but lies 

 as a level layer underneath mesoblast. 



It {i.e., hypoblast), coincident with the appearance of head and 

 tail folds, &c, becomes invaginated, and thus forms the primative 

 alimentary canal, closed in front and behind. Meanwhile, that por- 

 tion of the mesoblast lying on each side of notochord, beneath 

 medullary folds, split into two layers — one attaches itself to the 

 epiblast, forming with it the body-walls ; the other attaches itself 

 to the hypoblast, and assists it in forming the walls of alimentary 

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