516 A. A. W. HUBRECHT. 



however, seem that the process is most active in the region 

 posterior to the gastrula ridge. Even after mesoblast has 

 been derived from it^ the annular zone of hypoblast has a dif- 

 ferent aspect from the circular patch of hypoblast which it 

 encloses, as a glance at fig. 68, PI. XL, will reveal. 



There can hardly be a doubt that the annular zone of raeso- 

 blast-formation here described is homologous with the annular 

 mesoblast which Bonnet has described and figured in the 

 early developmental phases of the sheep. 



Whereas the embryos 73 have allowed us to analyse the 

 first origin of the mesoblast, the embryos 45 reveal a further 

 step in the development of the middle germinal layer, which 

 is very instructive. As far as the protochordal plate is con- 

 cerned, we have already above (p. 512) noticed in what respect 

 its features are changed when compared with stage 73, It is 

 not less important to observe that the forward growth from 

 the front end of the gastrula ridge which is often indicated as 

 the " Kopffortsatz " of the gastrula ridge, but which has here 

 been designated as the protochordal wedge, has evidently, 

 during its forward growth, carried forward with it the wings 

 of mesoblast. In stage 73 the protochordal wedge was as yet 

 only in its very first phases, and the phenomenon here alluded 

 to not yet very distinct. In the embryos 45 (figs. 68, 72 — 74) 

 lateral plates of mesoblast are seen to be confluent with the 

 median thickened string of tissue, the protochordal wedge. 

 The latter fuses with the hypoblastic protochordal plate, thus 

 constituting a continuous band of tissue, from which in the 

 further stages the notochord will develop. At the same time 

 the mesoblastic wings of protochordal wedge and gastrula 

 ridge fuse with the lateral mesoblast that has sprung from the 

 protochordal plate, and thus one continuous sheet is formed, 

 which in the reconstructed surface views of PI. XL, figs. 62 — 

 64, is again indicated by a dotted line. This dotted line marks 

 off the outer boundary line of the sheet of mesoblast; it is 

 again seen to be more or less circular, though, as we have seen, 

 it is not simply the increase of the mesoblast within the circle 

 of figs. 33 — 35 that has given rise to it, but also the anterior 



