636 A. A. W. HUBREOHT. 



when the formation of the notochord and mesoblast has 

 definitely commenced ; n. p. is the front end of the gastrula 

 ridge, and marks the dorsal lip of the blastopore. When a 

 neurenteric duct is present it is here that its dorsal opening is 

 situated. The epiblast of the embryonic shield is dark black, 

 which in the pre-blastoporian region (e.) is applied as a uniform 

 tint, in the region of the gastrula ridge interrupted by hori- 

 zontal white stripes. 



From the front end of the gastrula ridge a forward growth 

 inserts itself between epiblast and hypoblast ; it is our proto- 

 chordal wedge (Kopffortsatz, auct.), and is here marked by a 

 black band with round white dots. 



The lateral border of the epiblastic embryonic shield is indi- 

 cated in semi-perspective by a black boundary line, its general 

 surface by a light grey tint, which also marks the trophoblast 

 that forms the outer wall of the blastocyst. 



Turning to the hypoblast, we find it represented by a blue 

 tint clothing the inner surface of the blastocyst, forming a 

 continuous layer beneath the embryonic shield. Phenomena 

 of fusion between the blue hypoblastic lining of the blastocyst 

 and the palingenetic hypoblast of which the protochordal 

 wedge and the gastrula ridge consist, are in this phase already 

 apparent, but not marked in the diagram. A blue semi-perspec- 

 tive annular band [hy. az.) indicates the annular region of the 

 hypoblast that was fully described above, and that takes part 

 in the formation of mesoblast. A thickened patch of hypo- 

 blast, enclosed within the anterior border of this ring (our 

 protochordal plate), is represented in this longitudinal section 

 by a blue baud marked by darker blue round dots and con- 

 tiguous to the black band of the protochordal wedge.^ 



The mammalian diagram having thus been explained in its 

 general outlines, we shall have to consider how we can bridge 

 the considerable gulf that separates it from the diagram 92 of 

 the lower Ichthyopsida from which we have started in our 



' For better interpretation of tlie diagrammatic section, comparison with 

 the figs. 33 — 35, G2 and G4, in which tlie blastocyst is seen from above, will 

 be useful. 



