EARLY ONTOGENETIC PHENOMENA IN MAMMALS. 59 
ventrally, and is found back by us in this same position, and 
at the same time quite marked in Fig. 111 for Chameleo. 
For Lacerta all this is yet more distinctly brought before 
us in Wenckebach’s well-known figures (’86) reproduced in 
Hertwig’s Figs. 437—441. 
And as to birds Schauinsland’s longitudinal section of the 
sparrow (Fig. 110) leaves no doubt whatever that the 
phenomena are, indeed, the same here as in reptiles and 
mammals. Protochordal plate, protochordal wedge, lateral 
lips of the dorsal mouth (primitive streak), and ventral 
mesoblast have each their proper position, and their adequate 
further development. 
It is especially evident in this last-named section that the 
so-called Sichelrinne has the ventral mesoblast immediately 
behind it, and that in this region there is another thickening 
of the entoderm, preceding the formation of vasifactive tissue 
in this region as is indicated in the diagram of Fig. 46 that 
was more especially designed for mammals. 
In finding all this amount of correspondence between 
Mammals, Amphibia and Sauropsids concerning certain 
general features of the very earliest development of meso- 
blastic structures it was, of course, a great desideratum to 
know in how far the oviparous mammals, the Ornithodelphia, 
agreed. This paragraph was already written, and the blank 
caused by our ignorance on this head was keenly felt by me 
when the very recent memoir by Wilson and Hill on Ornitho- 
rhynchus ontogeny appeared in the ‘ Philosophical 'T'ransac- 
tions’ (07). This important paper furnishes us with the 
necessary data to fill up that blank, and I was only too pleased 
to have to re-model this paragraph as these data confirm in 
many respects the views here advocated, and accentuate a 
few points in my interpretation with quite unexpected deci- 
siveness. 
I may begin by remarking that a protochordal plate, of 
which mention has been made in the foregoing pages, is 
recognised by them as occurring in Ornithorhynchus, and is 
also designated by that name. However, the data concerning 
