CH. LVIII.] 



THE BLASTODERM. 



809 



The mesoblast, however, is not exclusively epiblastic in origin, 

 for some of the cells in the mass d, fig. 621, are doubtless 

 hypoblastic. Moreover, certain large 'formative cells' seen in 

 fig. 620 M and in the next figure (fig. 622), originate from hypo- 

 blast and wander into the middle layer, and it is these cells 

 which give origin to the connective-tissues and blood-vessels. 



m.g. 

 d.r. 



Fig. 622. Vertical section of blastoderm of chick (ist day of incubation). S, epiblast. 

 consisting of short columnar cells ; D, hypoblast, consisting of a single layer of flattened 

 cells; 31, "formative cells." They are seen on the right of the figure, pasting in 

 between the epiblast and hypoblast to contribute to the mesoblast; A, white yolk 

 granules. Many of the large "formative cells" are seen containing these granules. 

 (Strieker.) 



The three layers of the blastoderm show from the first 

 distinctive characters ; the epiblast and hypoblast present the 

 appearance of epithelium, whereas the mesoblast is composed of 

 cells which are not arranged close 

 together, and many of them are 

 branched. 



The primitive streak and groove 

 are evanescent structures ; they 

 indicate the longitudinal axis of 

 the embryo, but they are soon 

 replaced by a new and larger 

 groove. This is formed by two 

 new thickenings of epiblast which 

 rise up like walls on each side of 

 the middle line ; they are united 

 together in front, and they extend 

 backwards, enclosing and then 

 (fig. 623) obliterating the primitive 

 groove. 



These two walls are called the 



medullary plates or the dorsal ridges ; the valley between them is 

 called the medullary groove ; this is the first appearance of the 

 central nervous system. They approach one another, and meet in 

 the middle line, and so convert the medullary groove into a canal. 



Fig. 624 shows this diagrammatical ly in transverse section. 



The epiblastic cells which line the medullary canal are, by the 

 union of the two dorsal ridges, entirely cut off from the surface 



Fig. 623. Embryonic area of a rabbit's 

 ovum (7 days), pr., primitive 

 streak and groove ; m.g., medullary 

 groove ; d.r., donal ndge. (After 



