EARLY ONTOGENETIC PHENOMENA IN MAMMALS. 37 
have called the “ Kopffortsatz.” On the contrary, the 
moment the fusion with the protochordal plate has come 
about, a process of growth sets in of the tissues here con- 
sidered, not in a proximal, but in a distal direction. Asa 
comparison of Figs. 48, 52, 98, and 99 shows, the embryo- 
nic shield increases in length, and at the same time the dis- 
tance between the spot where the protochordal wedge has 
originated and the front end of the ectodermal shield becomes 
more considerable. But during this process the situation of 
the point of fusion between protochordal plate and proto- 
chordal wedge may be said to be more or less constant 
(though not actually any longer discernible), whereas both 
plate and wedge have increased in length at a relatively 
equal rate (see Figs. 95—95). And so the protochordal wedge 
becomes undoubtedly lengthened, not, however, by its send- 
ing out any “ Fortsatz,’ but by its being, so to say, “ spun 
out” in consequence of the backward growth of the tissue 
that is going to be the notochord,! thanks to new ectodermal 
proliferation being added to what had previously come into 
existence, and had fused with the entodermal protochordal 
plate. A thin canal is noted in mammals in the posterior 
part of the backward proliferation of this protochordal 
wedge (Figs. 98 and 99, ne, cn). “ 
b. The Ventral Mesoblast.—We will now for a moment 
leave the protochordal wedge and inquire whether, besides 
this, any further contribution of the shield ectoderm towards 
the formation of tissues between it and the endoderm takes 
place. 
In this respect T'arsius has proved to be a genus of mam- 
mals, which is of the utmost importance in throwing light 
upon these much disputed questions. Monkeys and man— 
1 T am inclined to think that, if all those investigators that have stood up 
so decisively for a forward growth of the mammalian “ Kopffortsatz” in other 
genera of mammals, were once more to look closely at their preparations, they 
would be willing to leave the possibility open that this forward growth may 
also in their case be an elongation, by material being added posteriorly, con- 
comitantly with the inerease in length of the shield, 
