i S 8 



FORMATION OF MESOBLAST. 



from a differentiation of the primitive hypoblast. I am however inclined to 

 believe that some of the mesoblast cells of the embryonic region have the 

 derivation which Kolliker ascribes to all of them. 



FIG. loo. TRANSVERSE SECTION THROUGH THE EMBRYONIC REGION OF THE 



BLASTODERM OF A CHICK AT THE TIME OF THE FORMATION OF THE NOTOCHORD, 

 BUT BEFORE THE APPEARANCE OF THE MEDULLARY GROOVE. 



ep. epiblast ; hy. hypoblast ; ch. notochord ; me. mesoblast ; n. nuclei of the 

 germinal wall yk. 



As regards the mesoblast of the primitive streak, in a purely objective 

 description like that given above, the greater part of it may fairly be de- 

 scribed as being derived from the epiblast. But if it is granted that the 

 primitive streak corresponds with the blastopore, it is obvious to the com- 

 parative embryologist that the mesoblast derived from it really originates 

 from the lips of the blastopore, as in so many other cases ; and that to 

 describe it, without explanation, as arising from the epiblast, would give an 

 erroneous impression of the real nature of the process. 



The differentiation of the embryo may be said to commence 

 with the formation of the notochord and the lateral plates of 

 mesoblast. Very shortly after the formation of these structures 



FIG. 101. TRANSVERSE SECTION OF A BLASTODERM INCUBATED FOR 18 HOURS. 



The section passes through the medullary groove me., at some distance behind its 

 front end. 



A. epiblast. B. mesoblast. C. hypoblast. 



m.c. medullary groove; m.f. medullary fold ; ch. notochord. 



the axial part of the epiblast, above the notochord and in 

 front of the primitive streak, which is somewhat thicker than 



