EKARLY ONTOGENETIC PHENOMENA IN MAMMALS. 85 
that the surface of the umbilical vesicle should, during the 
embryonic period, play an active part in this direction, if we 
remember how copiously blood formation is going on during 
embryonic life in another derivate of the entoderm, viz. the 
liver, not to mention the increased significance which we have 
to ascribe to the entoderm as the primary source of blood 
and blood-vessels, since the recent researches of Riickert and 
Mollier (06). One of the main arguments of Hd. van Beneden 
(99, p. 333) for not accepting my views on the phylogeny of 
the Mammalia was this, that the presence and the consider- 
able development of the umbilical vesicle of the Mammalia 
could according to him only be explained, if for mammals we 
accept a reptilian ancestor with meroblastic, yolk-laden eggs. 
Van Beneden’s reluctance might dwindle away if this hema- 
topoietic significance of the surface of that part of the ento- 
derm which has grown out into an embryonic appendage, and 
has been styled the umbilical vesicle, was found to be its 
chief “‘ raison d’étre.” 
That this hypothesis is not specially intended to stand as 
an argument against van Beneden’s criticism follows from 
Spee’s independent advocacy (’96) of the hematopoietic sig- 
nificance of this dense, vascular, network on the human (and 
simian) umbilical vesicle (Figs. 74, 156, 157). 
The question once again arises if this significance of the 
vascular area on the umbilical vesicle of the higher Verte- 
brates is not older than that other property which it has 
assumed in the meroblastic eggs of Sauropsida, viz. the pro- 
poietic processes is not yet’present, and that also the liver cannot be said as 
yet to furnish a sufficient number of blood-corpuscles for assisting in the 
metabolic processes of the Primate embryo. And so the development and the 
increase of an extensive hematopoietic network on part of the intestinal 
surface (which from the very first had its significance as matrix for vasifactive 
mesenchyme) is quite natural. Thus, a hernia-like expansion of part of the 
cut would first have had a hematopoietic signilicance (Primates), would then 
have secondarily acquired significance in omphaloidean placentation (many 
mammals), and would finally have co-operated (Sauropsida) towards the 
transport of a reserve of yolk-substance, which in these cases had become 
accumulated against the inner surface of this network. 
