150 A. A. W. HUBRECHT. 
I will in this chapter attempt to draw the conclusions con- 
cerning the systematic arrangement of the vertebrates as 
such, to which due consideration of all the facts here con- 
sidered must lead us, giving at the end a short sketch, partly 
already contained in earlier publications (’02, 05), of what 
may be considered as the most probable hypothetical inverte- 
brate ancestors to which all these views point. 
We have then first to take into account that the primary 
subdivision of the vertebrates is that ito the two great 
groups sharply defined against each other as the Amniota 
(Mammalia, Sauropsida) and the Anamnia (Ichthyopsida). 
It has long been known that parallel and identical to this 
subdivision another is possible into Allantoidea and Anal- 
lantoidea, and that the fact of the existence of this double 
character increased our faith in the significance of this 
primary subdivision of the vertebrates. 
However, we have since seen that it would be difficult to 
pretend that the Primates are true Allantoidea, a free allan- 
tois not being present in this order. And on the other hand 
we have seen that of even more importance than either 
amnion or allantois is the outer embryonic layer, the tropho- 
blast; in itself a larval envelope of very great antiquity. 
The trophoblast, which is most marked in mammals, is 
ever so much more hidden in Sauropsida, and its presence can 
here only be recognised by a careful comparison of all the 
variations which we notice in its relations to the embryonic 
epiblast respectively in Mono-, Di-, and Ornithodelphia. 
Clearer, however, than in most Sauropsida are certain remi- 
niscences of the trophoblast in many Amphibia, Dipnoi, and 
Teleostomi. ‘T'hey consist in the presence, during early larval 
life, of an outer, generally somewhat more strongly pigmented, 
and also generally flattened layer of cells, which disappear 
when development proceeds, and which correspond, as far as 
modified views seem to them to be more acceptable than the current 
opinions on the vertebrate gastrulation. This agreement is all the more 
welcome as Keibel, by his comprehensive article in the ‘ Ergebnisse der Anat. 
und Enter. geseli.,’ vol. 10, has an authoritative voice in the matter. 
