538 A. A. W. HUBRECHT. 



this respect the comparison of fig. 92 with fig. 95 holds good. 

 It also does this if we compare the lower hemispherical hypo- 

 blastic surface of fig. 92^ which we fiud back in the diagram of 

 fig. 95 as a flat annular baud of hypoblast. This shape it 

 must necessarily have taken if we suppose the transitional 

 phase of fig. 93 to have yet further become bulged out^ so that 

 finally, as was already discussed on p. 528, the formative 

 blastoderm became spread out flat on the upper surface of a 

 much larger spherical blastocyst, as is indicated in fig. 95 . 



The identity of the regions marked by the blue and white 

 dots and by the thin white stripes in the figures is, moreover, 

 self-evident. 



And so now we have to turn to our hypothesis of precocious 

 segregation of part of the hypoblast, and see how it can be 

 applied to the diagrams here given. For this I have made use 

 of the diff'erent colours, and will first discuss the mammalian 

 diagram. The epiblast, whether of the embryonic shield or of 

 the trophoblast, is black or grey. So is the palingenetic hypo- 

 blast, which arises in the gastrula ridge and extends forwards 

 as the protochordal wedge. The blue sphere is the closed sac 

 of cenogenetic hypoblast, the constituent cells of which have 

 wandered inwards before the actual gastrulation process com- 

 mences. On this cenogenetic hypoblast certain modified por- 

 tions — the protochordal plate and the annular ring — appear 

 before the definite fusion between the palingenetic and ceno- 

 genetic elements has been accomplished. 



Those portions of the amphibian hypoblastic cell-mass, 

 which I consider to be homologous to the mammalian ceno- 

 genetic hypoblast, are in fig. 92 artificially distinguished from 

 the remaining part of the hypoblastic invagination by a similar 

 blue colour. The way in which I picture to myself that from 

 stage 92 the mammalian stage 95 has been arrived at was fully 

 discussed above. I have here only to add that in all these 

 diagrams a prominent part is allowed to the hypoblast in the 

 formation of the notochord. This is indicated by the dark 

 blue dots in the dorso-median region contiguous with the proto- 

 chordal wedge. Suppose for a moment that the confirmatory 



