186 K. M. BALFOUR AND i\ DEIGHTON. 



though retaining the same relation to the germinal layers as in 

 front. 



In the section immediately behind (Ser. k^ 4) the convex upper 

 surface of the notochord has become continuous with the epi- 

 blast for a very small region. The section, in fact, traverses the 

 front end of the primitive streak. 



In the next section the attachment between the epiblast and 

 the cells below becomes considerably wider. It will be noticed 

 that this part of the primitive streak is placed on the floor of the 

 wide medullary groove, and there forms a prominence known as 

 the anterior swelling of the primitive streak. 



It will further be noticed that in the two sections passing 

 through the primitive streak, the hypoblast, instead of simply 

 becoming continuous with the axial thickening of the cells, as in 

 front, forms a more or less imperfect layer underneath it. This 

 layer becomes in the sections following still more definite, and 

 forms part of the continuous layer of hypoblast present in the 

 region of the primitive streak. 



A comparison of this stage with the previous one shows very 

 clearly that the notochord is formed out of the median plate 

 of cells of the earlier stage, which was not divided into mesoblast 

 and hypoblast, together with the short column of cells which 

 grew forwards from the primitive streak. 



The notochord, from its mode of origin, is necessarily con- 

 tinuous behind with the axial cells of the primitive streak. 



The sections immediately behind the last we have represented 

 show a rudiment of the neurenteric canal of the same form as 

 that first figured by Gasser, viz. a pit perforating the epiblast with 

 a great mass of rounded cells projecting upwards through it. 



The observations just recorded practically deal with two much 

 disputed points in the ontogeny of birds, viz. the origin of the 

 mesoblast and the origin of the notochord. 



With reference to the first of these our results are briefly as 

 follows : 



The first part of the mesoblast to be formed is that which 

 arises in connection with the primitive streak. This part is in 

 the main formed by a proliferation from an axial strip of the 

 epiblast along the line of the primitive streak, but in part also from 

 a simultaneous differentiation of hypoblast cells also along the 

 axial line of the primitive streak. The two parts of the meso- 

 blast so formed become subsequently indistinguishable. The 

 second part of the mesoblast to be formed is that which gives 

 rise to the lateral plates of mesoblast of the head and trunk 

 of the embryo. Tiiis part appears as two plates — one on each 

 qide of the middle line-— which arise by direct differentiation 



