EARLY ONTOGENETIC PHENOMENA IN MAMMALS. 31 
In Sorex it was particularly interesting to be able to 
establish the independence of this early proliferation from 
any further source of mesoblast, although very soon after, the 
annular zone of mesoblast-producing entoderm connects this 
early protochordal plate with the mesoblast-producing regions 
at the hinder end of the embryonic shield. Seen from above 
this early phase is pictured in Figs. 59 and 60. 
The entodermal proliferation here described has, in its 
earliest phases, the aspect of a mere thickening of the lower 
germ-layer, but very soon that aspect changes, and we notice 
that certain of the proliferated cells break away from their 
place of origin, and take up a situation between the two 
germ-layers. ‘The extent to which the invasion by these 
mesenchyme cells of the space between ectoderm and ento- 
derm spreads cannot always be very strictly determined for 
two reasons. Firstly, because the annular zone of mesoblast- 
producing entoderm, which becomes confluent with the 
protochordal plate (Fig. 60), starts its activity almost 
simultaneously, though, as Fig. 59 shows, just a bit later; 
secondly, because another invasion of this space—starting 
from the ectoderm—also begins to form about this time, and 
will be described below. 
At a very early moment the cells derived from these three 
different sources intermingle, and it will prove a most intricate 
problem, which up to now has not yet been fully nor satisfac- 
torily solved, nay, not even approached, to make out from 
which of the three starting points the various organic tissues 
have ultimately been derived. 
In Sorex this was possible to some extent at a very early 
stage, because the anterior entodermal proliferation pro- 
ducing mesenchyme cells is inaugurated somewhat earlier 
than the process which starts from the posterior half of the 
ectodermal shield. In my paper on Sorex (’90) I have been 
able to sufficiently distinguish these early phenomena, 
although fully recognising that after a time further dis- 
crimination becomes impossible. This latter fact may have 
contributed to bring so many of the best modern embryolo- 
