512 A. A. W. HDBREOHT. 



recognise (though I have not, as in figs. 31 — 34^ marked this by 

 a darker shading) our protochordal plate^ broader and more 

 massive, and laterally merging into plates of mesoblast (figs. 66^ 

 67, 70, and 71). The cells of this mesoblast have no longer a 

 fusiform aspect, but are more rounded and similar to the meso- 

 blast-cellsin the middle and posterior regions of the embryonic 

 shield. A comparison of fig. 66 with 67, and of 70 with 71, 

 will show that in 67 and 71 it is more difficult to distinguish 

 between protochordal plate-cells, mesoblast-cells, and hypoblast- 

 cells than in 66 and 70, although these latter sections are 

 situated further forwards. It is in this stage that the proto- 

 chordal plate may be said to have reached its maximal exten- 

 sion. In the following stage (embryos 42, figs. 79 — 81) it is 

 hardly more than one or two cells thick ; the mesoblast has be- 

 come much more distinctly separate from it. In still later stages 

 the protochordal plate-cells are fairly on the way of becoming 

 indistinguishable for a time from the adjoining enteric hypo- 

 blast. With this they extend as a continuous plate (only one 

 cell thick) below the medullary groove or canal, and only later 

 the front portion of the notochord folds off, this being the last 

 phase in the developmental phenomena of what we have called 

 the protochordal plate. This has been figured by Heape for 

 the mole, and is not further entered upon in this paper. 



It is important to distinguish this primary excalation of noto- 

 chordal tissue in the anterior protochordal plate region from the 

 secondary excalation which takes place further backwards in 

 the region where the protochordal wedge has first fused watli 

 the enteric hypoblast. This latter phenomenon has been de- 

 scribed by other authors (7, 24, 29). 



Passing to the second source of mesoblast-cells, the gastrula 

 ridge, we find it to be indeed a very considerable contributor 

 to the increase of the middle layer. 



We have already noticed that it appears after the proto- 

 chordal plate has become very distinctly differentiated. 



When the gastrula ridge for the first time becomes apparent 

 (embryo 73/, figs. 32 and 37 — 39) we notice a fusion which 

 then obtains between hypoblast-cells (that have hitherto been 



