508 A. A. W. HUBEECHT. 



A most remarkable fact, to which I must novr call attention, 

 is this, that it is not in the posterior region of the epiblastic 

 shield that the formation of the middle layer and its earliest 

 representatives — notochord and lateral mesoblast plates — is 

 first inaugurated. It is in the hypoblast that the first 

 differentiation occurs which ultimately leads to the 

 formation of the above-mentioned structures. In the 

 dozen or more of embryos of this stage which are in my posses- 

 sion I invariably find that the hypoblast undergoes an 

 important modification always in the selfsame spot, i. e. just 

 below the anterior end of the embryonic shield. Here the 

 hypoblast, which from the beginning was never more than 

 one cell thick, thickens over an area which soon counts 

 some five or six dozen of cells. It is not at the outset a 

 splitting process, but the cells simply become thicker and more 

 massive (fig. 30), and lose their flattened fusiform shape, 

 whereas the nuclei become much more closely packed. 



In fig. 20 the hypoblast-nuclei underlying the epiblastic 

 shield are drawn in situ. This will give a general apprecia- 

 tion of the phenomenon, and at the same time prevent that the 

 dark patches in figs. 18 — 21 and 32 — 35, by which the region 

 here mentioned is indicated, should be regarded as very sharply 

 separated from the rest of the hypoblast. 



This patch of modified hypoblast-cells has at the beginning 

 an oval shape, with the long axis perpendicular to that of the 

 embryonic shield. Part of this patch will develop into the 

 anterior portion of the notochord ; for this reason I will call 

 it the protochordal plate. 



In the stages immediately consecutive upon this it retains 

 its isolated position in the rest of the hypoblast ; but now an 

 increase in thickness is noted by the cells dividing in a plane 



structions from an unbroken series of sections. If the plane of section is 

 not quite perpendicular to that of the embryonic shield, the reconstruction 

 will not exactly correspond to the shape of the latter, but rather to its pro- 

 jection on another plane. Still, with due allowance for similar small devia- 

 tions, the figures here given may be said to give an accurate representation of 

 the developmental phases of the embryonic shield. 



