III.] 



VERTEBRAL PLATE. 



69 



enlarges, while that in the vertebral plate remains in 

 the condition of a mere split. 



FIG. 24. 



TRANSVERSE SECTION THROUGH THE DORSAL REGION OP AN 

 EMBRYO OF THE SECOND DAY (copied from His), intro- 

 duced here to illustrate the formation of the mesoblastic 

 somiti^, and the cleavage of the mesoblast. 



M. medullary canal ; Pv. mesoblastic somite ; w. rudiment of 

 Wolffian duct; A. epiblast; C. hypoblast ; Ch. notochord ; 

 Ao. aorta ; BO. splanchnopleure. 



At first each vertebral plate is not only unbroken 

 along its length, but also continuous at its outer edge 

 with the upper and lower layers of the lateral plate 

 of the same side. Very soon, however, clear trans- 

 verse lines are seen, in surface views (Fig. 23), stretch- 

 ing inwards across each vertebral plate from the edge 

 of the lateral plate towards the notochord ; while a 

 transparent longitudinal line makes its appearance on 

 either side of the notochord along the line of junction 

 of the lateral with the vertebral plate. 



The transverse lines are caused by the formation 

 of vertical clefts, that is to say, narrow spaces containing 

 nothing but clear fluid ; and sections shew that they 



