64 A. A. W. HUBRECHIT. 
Amphioxus, we may term the essentially entodermic proto- 
chordal plate (pp) and the (ectodermic) protochordal wedge 
(pw). 
In Elasmobranchs the comparison is not either difficult or 
strained. We see in the Figs. 117—119 and 121—123, copied 
from His and Riickert, that when once the separation of the 
two primary layers has come about by delamination, a pro- 
liferation in the ectoderm makes its appearance (pw Fig. 122), 
which here, too, deserves the name of protochordal wedge, 
whereas the undoubted entodermal layer (pp) to the left of 
the figure is, and remains, the homologue of what im the 
mammals we have called the protochordal plate. In Fig. 128 
notogenesis is in its incipient stage; in Fig. 117 it has con- 
siderably advanced, and in 118 and 119, where the headfold 
has made its appearance, part of what was the horizontally out- 
spread layer (pp) has become lifted up from the yolk and is now 
that part of the entoderm which lies in front of the anterior 
end of the notochord, and which participates by means of its 
surface that faces the ectoderm in the formation of the endo- 
thelium of the heart (cf. Riickert and Mollier in ‘ Hertwig’s 
Handbuch? Bd. 1, 5" 1l,sp: 10573835754). 
Perfectly similar occurrences were formerly described by 
me (02, Pl. IX, figs. 72-74) for Tarsius, and Figs. 98, 99 
should be compared with those here given for the Elasmo- 
branchs. 
As to the presence in Elasmobranchs of a ring-shaped 
extent of entoderm which contributes to the formation of the 
mesenchyme, out of which the vascular system is going to take 
its origin, we find the most reliable data in Riickert and 
Mollier’s extensive treatise above cited. Their figure 777, 
here reproduced as Fig. 124, is especially instructive, and it 
will suffice to refer to that important article in ‘ Hertwig’s 
Handbook.’ It will there be seen that also by the presence 
of a circular region of mesenchyme-producing entoderm the 
early Hlasmobranch stages resemble the mammalian. Although 
in Riickert’s article a certain reluctance is unmistakable to 
admit the value of his conclusion as to the entoblastic origin 
