EARLY ONTOGENETIC PHENOMENA IN MAMMALS. 257 
resembles more closely the Avine embryo, such as a rabbit at 
168-178 hours, except that the mesoblast, especially of the 
anterior region, is much more abundantly developed. 
Wilson and Hill’s figs. 16 and 15, PI. 5, on this interpreta- 
tion pass through what Hubrecht would call the protochordal 
plate, showing mesoblast or mesenchyme, which shows no 
sign of having been derived from primitive streak. Thei’e is 
not a hint of any proliferation of cells from the thickened 
epiblast above. In respect of the mesoblast or mesenchyme 
development the condition is far more Avine (cf. sparrow) 
than Eutherian. 
So fig. 14 is perfectly typical of a transverse section 
through the front end of the primitive streak of bird or 
mammal at this early stage, i.e. when the primitive streak 
has attained its full length and before the counteracting- 
effects of the primary and secondary growth centres have 
reached their highest development. 
In fact there are only slight details which distinguish it 
from an embryonal area typical of those that develop upon a 
vesicle tense under hydrostatic pressure. 
4’here is nothing at this stage to suggest any connection 
between this embryonal area and the curious mass of cells 
and yolk, pr. K. (Wilson and Hill’s “ primitive knot”). 
Why should not this embryonal area develop according to 
the plan upon which, say, the sparrow embryonal area 
develops in which a neurenteric canal is produced though 
not quite in so marked a manner (vide Schauinsland’s figure, 
Hubrecht, fig. 1 10) ? 
In text-fig. 8 the next stage is illustrated, by which time, 
according to Wilson and Hill, the embryonal area of text-fig. 
7 has expanded on the whole symmetrically so that the 
general outlines of mesoblast, thickened epiblast, and area 
opaca retain the same general relations to one another, the 
primitive streak, however, becoming slightly shorter, and 
occupying a i-elatively more posterior position. 
'I’his is exactly what occurs in similar types of embryonal 
areas in birds or mammals like rabbits. Compare, for instance. 
