EARLY ONTOGENETIC PHENOMENA IN MAMMALS. 13 
express it in O. Hertwig’s own words: “ Die gréssten 
Schwierigkeiten bereitet den Embryologen die Keimblattbil- 
dung bei den Situgethieren. . . wegen der von anderen 
Wirbelthieren stark abweichenden Befunde” (’06, p. 898). 
As soon as we separate the phenomena of notogenesis, 
such as they are found in all vertebrates — Amphioxus 
included—from the phenomenon of gastrulation, recognising 
that the former follow upon the latter and bring about 
the formation of the notochord and the mesoblastic somites, 
the difficulties are considerably simplified. 
Gastrulation is thus terminated in the mammalia when the 
didermic stage of the embryonic shield has come into exist- 
ence. We have seen that this takes place not in conse- 
quence of any process of invagination but by means of a 
most unmistakable delamination of the entoderm, out. of 
the embryonic knob. 
This delamination gastrula of the mammalia generally enters 
upon the later phases of ontogeny which will be described 
hereafter without the appearance of a distinct blastopore. 
Still to this there are a few notable exceptions that have 
gradually come to light, but have been mostly overlooked 
or misinterpreted in consequence of the erroneous views 
above alluded to. The most striking example is undoubtedly 
offered by the hedgehog, where the blastopore, a clearly 
visible perforation towards the hinder end of the embryonic 
shield, makes an evanescent appearance at one. particular 
stage of the individual development (Fig. 53). 
Along the lips of this opening the ectoderm and entoderm 
pass into each other, whereas these two layers, although 
genetically related, have up to this moment been separated 
and nowhere in confluence with each other. This latter fact 
is recognised by all observers. Iam inclined to believe that 
the formation of the blastopore in the hedgehog is not only 
very evanescent, but that it does not necessarily appear in 
all hedgehog-embryos, and that in exceptional cases the 
formation of notochord and somites may commence without 
the blastopore having become a visible opening. 
