EARLY ONTOGENETIC PHENOMENA IN MAMMALS. 239 
at an extremely early age (Kopscli, ’01, Assheton, ’08), thus 
resembling the Eutherian trophoblast. 
The Teleostean Deckschicht differs from those of the 
Amphibia and Dipnoi in being quite free from any connec- 
tion with the lips of the blastopore, over which it passes as a 
continuous layer, e. g. Gymnarchus, Salmo. 
In view of the uncertainty of its presence in Sauropsida, 
of its widely diffei'ing character and relations among Amphibia, 
Teleostomi and Dipnoi, of its unique character and function 
in Eutherian mammals, of the dubious nature of its existence 
in Monotremes and Marsupials, is it not rash to regard all 
these outer layers as homologous, and as constituting a 
feature of such importance as to justify the abandonment of 
the old group Amniota and the formation of a totally new 
association, having this feature as the chief diagnostic 
character? Anyhow, the points to which I have drawn 
attention seem to me to deserve further consideration. 
Another point I might make here. On p. 106 Hubrecht 
writes, with reference to his very remarkable observations 
published in 1899 on the blood formation in the Tarsius 
placenta: “The production of blood-corpuscles by the cells 
of a larval envelope is surely an unexpected histological 
phenomenon. Still, the details of differential segregation 
during the successive stages of cell lineage are not yet well 
enough known to justify any apodictic negation. The 
possibility is not excluded that at the first cleavage (suppose 
this to separate trophoblast from embryonic knob) certain 
potentialities of haematogenesis may be passed on to this 
trophoblast mother cell.” 
On the hypothesis that the Eutherian trophoblast is really 
of yolk-cell or hypoblast origin, the formation of blood- 
corpuscles from it is much less extraordinary, for it is from 
this layer that the first formed blood-cells arise in Sauropsidan 
and other vertebrate embryos. 
