EARLY ONTOGENETIC PHENOMENA IN MAMMALS. 61 
Ornithorhynchus—for the details of which I refer the reader 
to Wilson and Hill’s paper (’07)—that in that particular 
region of the embryo, where notogenesis comes about, there 
are multiple centres of growth. What was in the pelagic 
vermactinian stage of the vertebrate ancestor (Fig. 160) 
the dorsal mouth-slit or ‘“ Riickenmund” (from which the 
stomodzeum [notochord] contimued downwards towards the 
intestine while the enteric chambers preceded the ccelom) has ~ 
left in the early/vertebrate embryo hereditary traces of its 
gradual extension backwards and of its closure.’ ‘The proximal 
end of this dorsal mouth-slit is the earliest protochordal wedge, 
the distal end of it is our earliest growth-centre of the ventral 
mesoblast. Between these two there is (1) a backward 
growth of the protochordal wedge (above discussed for 
Tarsius and Amphibia) ; (2) a forward growth going to be 
confluent with the preceding and established by Wilson and 
Hill for Ornithorhynchus, as well as (3) lateral expansions 
from what may be called the lateral lips of the dorsal mouth- 
slit. What has been called the “Sichelrinne” is always 
situated at the distal end of this medio-dorsal region of pro- 
liferation, whereas what is originally the protochordal wedge 
(Hensen’s knob) is always at the proximal extremity in the 
original stage. It may be said to travel for a certain distance 
backwards before becoming unrecognisably united to the 
posterior proliferation. 
What has sometimes been called the archenteric cavity in 
the protochordal wedge (which has been compared with the 
archenteron of Amphioxus by van Beneden, who has more 
especially studied it in bats [’87]), what has been termed Meso- 
dermsickchen by O. Hertwig (’06, p. 828), and what has been 
found as a transverse slit in Ornithorhynchus by Wilson and 
Hill, and as a decided cavity by many other authors, such as 
Will (90), Mitsukuri (93), Ballowitz (’01), Wenckebach (786), 
1 In this respect Hertwig’s views can be made to fit in very well with mine, 
only with the difference that the “ Riickenmund” is not to be confused with 
an “ Urmund,” and that ‘‘ notogenesis ” is not to be looked upon as “ gastru. 
lation.” 
