BKARLY ONTOGENETIC PHENOMENA IN MAMMALS. 35 
spring from this entodermal proliferation will require very 
close investigation in all the different orders of Mammals. 
2. Developmental Processes in the Ectoderm. 
I have on purpose postponed the discussion of the processes 
of proliferation in the ectoderm because in modern text- 
books those in the entoderm are generally ignored or even 
denied, whereas as a matter of fact they are antecedent, at 
least in their very earliest appearance, to those which con- 
cern the ectoderm. 
About the latter a very stately list of investigators, including 
many of the foremost embryologists, have published the results _ 
both of observation and of reflexion. Still we cannot say that 
at present a general consensus of opinion concerning these 
processes has been arrived at. They have been very ably 
summarised by O. Hertwig in his “Lehre von den Keim- 
blattern” (703, pp. 918—940), and to that author I would 
direct those who are interested in the historical development 
of the different views held on this point. 
This will give me occasion to skip at the present moment 
all controversial matter, and will allow me to put forward my 
own view of the case based on the examination of numerous 
early stages of different mammals. The point of divergence 
with other authors will then be noticed only afterwards. 
a. The Protochordal Wedge.—At the time when the 
two germ-layers of the round or oval embryonic shield are not 
yet interlocked, but independent of each other, and when the 
future front region of that shield can already be distinguished 
by the proliferation in the endoderm noticed in the preceding 
paragraph, and many years ago designated by me (’90) by the 
name of protochordal plate, a proliferation, directed down- 
wards, of the ectoderm in the axis of the embryonic shield 
and somewhere in the posterior third of it, becomes visible. 
I have no hesitation in saying that this spot coincides with 
the anterior lip of what was described in Chapter II as the 
