III.] 



THE PKOTOVERTEBR^. 



55 



but clear fluid ; and transverse sections shew that they are 

 due to breaches of continuity in the mesoblast only, the 

 epiblast *and hypoblast having no share in the matter. The 

 first transverse lines which appear are two in number, one a 

 little behind the other, about opposite the spot where the 

 medullary folds first coalesced to form the neural tube. The 

 longitudinal lines begin at about the same place and run thence 

 backward, parallel to the notochord, as far as the closure of 

 the medullary canal extends. Behind the first two transverse 

 lines other parallel transverse lines in course of time make 

 their appearance. 



Thus each vertebral plate appears in surface views to be 

 cut up into a series of square plots, bounded by transparent 

 lines. Each square plot is the surface of a corresponding 

 cubical mass (Fig. 13, F. v., Fig. 20, P. v.). The two such 



Fig. 13. 



,-m: 



.J^ ..£, 



\Ch 



'AO \I3C \jPp 



Teansveese Section through the Dorsal Region of an Embryo op the 

 Second Day. (Copied from His), introduced here to illustrate the forma- 

 tion of the protovertebrae, and the cleavage of the mesoblast. 



The provertebree appear irregularly quadrate ; they would have been more 

 distinctly square, had 1;he section been one of the fiist day. before the appear- 

 ance of the primitive aortse, and of the rudiments of the Wolffian ducts : com- 

 pare Fig. 20. 

 M. medullary canal. Pv. protovertebra. w. rudiment of Wolffian duet. 



A. epiblast. C. hypoblast. Ch. notochord. Ao. aorta. BC. splanch- 



nopleure. 



cubical masses first formed, lying one on either side of the 

 notochord beneath, and a little to the outside of the medul- 

 lary folds, are the first pair of protovertebrce. Behind this 

 first pair, but otherwise similarly situated, a second and third 

 pair make their appearance during the first day. 



