60 A. A. W. HUBRECHT. 
the earliest appearance of this protochordal plate in Ornitho- 
rhynchus are too scanty than that I have ventured to mention 
it when in the preceding pages we discussed the protochordal 
plate. And it seems advisable on this point to await yet 
further researches on these rare mammals, of which it is so 
very difficult to obtain the required developmental stages. 
As to the protochordal wedge and the ventral mesoblast of 
Sauropsida and Ornithodelphia some startling facts have 
been brought to light by Wilson and Hill, and will here be 
compared to what we find in reptiles, ‘birds, and mammals 
higher than the monotremes.} 
The most important discovery in Ornithorhynchus seems 
to be that mesoblast formation on the upper surface of what 
corresponds to the mammalian embryonic shield is started at 
two different spots lying in the median line. In other 
words the protochordal wedge and. the ventral 
mesoblast. which we have followed in all the’details of 
their earlest origin in Tarsius (p. 36), and which we have 
there found in closest proximity (see also Figs. 48, 49, and 50) 
are in Ornithorhynchus as wide apart as is indicated by 
Fig. 115. In Sauropsids, again, they have been found con- 
fluent by successive authors, but then in Ornithodelphia they 
become confluent soon after (Fig. 116), so that there is yet 
room for inquiry whether perhaps certain Sauropsids might 
not in very early stages conform with Ornithorhynchus. 
The lesson should be drawn from the arrangements in 
1 In passing, I may remark that Wilson and Hill’s article is a very instruc- 
tive example of the impossibility we have come to of retaining the current, 
nomenclature, as this has gradually developed itself out of successive contribu- 
tious of different authors. Only gradually can we attempt to obtain a more full 
comparative grasp of the subject here treated, and then such names as head- 
process, primitive knob and streak, and many other, prove to be a misleading 
encumbrance. Already Wilson and Hill do not look upon the primitive 
streak of mammals and reptiles as a homologous structure (’07, p, 116), 
whereas they propose to drop “head-process”’ altogether (a point already 
advocated by myself and others). But then they add new ones, such as 
archenteric plate and others, the acceptability of which will yet have to be 
tested and seems to me questionable. 
