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EICHAKD ASSHETON. 
former ploughed its way back so as to join up with the 
anterior end of the primitive streak ? This is incredible. 
Has the primitive streak actually migrated forward to 
meet the knot ? 
Unless it has been accompanied by the mesoblast there is 
no evidence of a possible advance beyond 8 mm. and the 
distance to be traversed is 22 mm. (in the figures). 
Has the primitive streak stretched or differentiated 
forwards ? 
No, it is actually shorter by 4 mm. than it was. Surely it 
is simpler to conclude that there has been no junction, but 
that the embryonic area has developed in an ordinary fashion 
like other mammalian embryonic areas, and that the so-called 
archenteric knot was really the remnant of an almost liquefied 
yolk-mass, and has now become so much further reduced as 
to be unrecognisable. And its position is, as before, well in 
advance of the anterior border of the embryonic ai-ea. 
When we come to look at the photographs of the sections 
of the archenteric knot in the post-gastrula stage and compare 
them with the gastrula stage we notice that the histology is 
very different, for, instead of a mass of yolk with few small 
nuclei we have a quite different tissue, a compact cellular 
tissue crowded with nuclei. 
To sum up, my suggestion is as follows : 
(1) The primitive knot is the remains of the reticulum 
which contained the yolk mass. 
(2) The “primitive knot,” therefore, forms no part of the 
so-called embryonal area. 
(3) Gastrulatiou occurs as in other Amniotes by infiltration 
of fluid forming a subgerminal cavity. 
(4) The embryonic area of the authors, text-fig. 7, expands 
into that of text-fig. 8, as corresponding ones do in mammals 
like pig or rabbit, or in birds. 
(5) The mass of mesoblast lying in front of the primitive 
streak of text-fig. 7 is the protochordal plate which differen- 
tiates into the anterior part of the vascular system, notochord 
(Wilson and Hill, archeuteric plate), and mesoblast. 
