AVES. 131 



shew for certain by sections whether or not there is a growth such as he 



n. 





FIG. 100. TRANSVERSE SECTION THROUGH THE EMBRYONIC REGION OF THE BLASTO- 

 DERM OF A CHICK AT THE TIME OF THE FORMATION OF THE NOTOCHORD, BUT BEFORE THE 

 APPEARANCE OF THE MEDULLARY GROOVE. 



ep. epiblast; hij. hypoblast ; cli. notochord ; me. mesoblast; >i . nuclei of the germi- 

 nal wall yk. 



describes ; but such sections as that represented in tig. 99 (and I have 

 series of similar sections from several embryos) appear to me to be conclu- 

 sive in favour of the view that the mesoblast of the region of the embryo 

 is to a large extent derived 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 Knlliker ascribes to all of 

 them. 



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 the axial part of 

 the epiblast, above the notochord and in front of the primitive streak, 

 which is somewhat thicker than the lateral parts, becomes differ- 

 entiated into a distinct medullary plate, the sides of which form two 



me 



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

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

 front end. 



A. epiblast. B. rnesoblast. C. hypoblast. 



iii.i-. medullary groove; m.f. medullary fold; </;. notochord. 



9 



