415 



more densely stained and appear more elongated than those of the 

 central mass; and they are approximately parallel, while the nuclei 

 of the central mass, except some at the base, are without definite 

 arrangement. This central mass is the chorda-hypoblast near the end 

 of the process of folding. It is flanked by hypoblast. The cells at 

 the base of the chorda-hypoblast near to the hypoblast are hypo- 

 chordal cells which have not yet changed their shape and retain there- 

 fore the primitive disposition: their nuclei appear elongated and they 

 are parallel to one another. There is an artefact in the form of a 

 cleft at one side of the chorda-hypoblast, running into it. 



The tracing of the chorda-hypoblast towards the posterior end, 

 and the proving of its identity with the whole of the archenteric roof 

 between the mesoblastic rudiments, and not only a part of it, was a 

 matter of extreme difficulty. And I am not at all confident about it, 

 in spite of confirmatory evidence obtainable elsewhere. 



As we follow the chorda-hypoblast forward from the above section, 

 it soon becomes impossible to demarcate its boundary exactly in every 

 section, for histological specialisation of its cells is confined to the 

 posterior end. But there is enough to guide us upon the essential 

 point: that it passes upwards and that its place in the dorsal wall of 

 the gut is taken by hypoblast cells. Among other features there is 

 the contrast in apparent shape; and yet, 

 though the hypochordal cells change their 

 shape, some of the hypoblast cells do so 

 temporarily. And there is also a wedging 

 apart of the hypoblast cells (cf. Fig. 5). 

 At the umbilical region we come to the 

 posterior end of the split-ofi" notochord, 

 with the remnant of the chorda-hypoblast 

 below (Fig. 3, where the wedging-apart of 

 hypoblast cells is also seen). The thinning 

 out of the hypoblast will be noted; and 

 further forward in the more advanced part 

 of the embryo, where the hypochordal cells 



are above the hypoblast, this has disturbed the shape of the hypoblast 

 cells and has caused the hypochordal rudiment to spread out instead of 

 being ridge-like (Fig. 4, in which four nuclei are hypochordal). Passing 

 to the head we meet again with primitive conditions, for the hypochordal 

 cells dip more and more into the hypoblast and lessen in number, 

 to disappear at a point just short of the anterior ends of gut and 

 notochord, a point beyond which they naturally do not extend. What 



Fig. 3. 4 mm embryo. 

 Through umbilical region. 

 X 400. 



