j:akly ontogenetic phenomena in mammals. 259 
Also the two structures are unlike in other ways. The 
primitive knot appears not very different from a yolk-laden 
reticulum with few nuclei^ while the histological characters 
of what is alleged to be the same spot of the later stage 
present the crowding of nuclei usual to the front end of the 
primitive streak. 
It is hardly likely that so fundamental a process as the 
formation of the neural plate — an organ whose formation in 
any other vertebrate is the earliest of all organs — should be 
represented by a squamous epithelial layer at a time when the 
primitive streak and mesoblast is fully established; especially 
unlikely is it when at this very time a plate of epiblast com- 
parable in all respects, form, position, and time of origin, to a 
normal neural plate, already exists in its correct position, as 
shown by figs. 16 and 15 in PI. 5, and text-fig\ 7 of Wilson 
and Hill’s ' Phil. Trans. ’ paper. 
The next stage, called by Wilson and Hill post-gastrular, 
is represented by four eggs, E, F, P, and PP. The diameter 
of the first-named was 12’5 mm. 
This stage is represented by their text-fig. 8. 
In this it is supposed that the embryonic area has expanded 
in such a way as to include now the primitive knot. 
In text-fig. 8 primitive knot, henceforth called by the 
authoi’S the “archenteric knot,” is shown as abutting upon 
the front end of the primitive streak. But the primitive 
streak itself has not increased in length, in fact it has slightly 
diminished. 
In fig. 7 the archenteric knot is 22 mm. in front of the 
primitive streak. 
In fig. 8 it touches it. 
lu text-fig. 7 the primitive streak is 39 mm. in length, in 
text-fig. 8 it measures 35 mm. 
In text-fig. 8 the posterior end of the primitive streak is 
8 mm. further off the posterior limit of the mesoderm than in 
text-fig. 7. 
How can the junction of the archeuteric knot and anterior 
end of primitive streak have been brought about ? Has the 
