56 A. A. W. HUBRECHT. 
little more closely before affirming that the same can be said 
of the annular mesenchyme-producing zone in the entoderm. 
If we consult Mehnert’s article in which he discusses the 
origin and the development of the hzmovasal tissue (area- 
vasculosa-crescent) in Hmys and Struthio (’96) we will then 
find that for these two Sauropsids he accepts as the final 
point of origin of the vascular tissue and the blood the ento- 
derm. But he does more than that. He gives a detailed 
account which in most points corresponds most exactly with 
what we have above indicated for the mammalia, of the 
origin in the hinder part of the “ primitive streak” of a 
decided entodermal proliferation which by many authors 
has been incorrectly looked upon as ectodermal. I believe 
that a careful re-examination of their preparations and a 
comparison of those with the numerous section series of 
Tarsius and Tupaja, which are always available for that 
purpose, may convince even those who have formerly stuck to 
the purely ectoblastic nature of the primitive streak, that in 
the lower half of the primitive-streak-tissue a direct and 
considerable proliferation of entoderm cannot possibly be 
denied. This proliferating region is, as we have seen in 
mammals, nothing else but the hinder median portion of 
the ring of vasifactive tissue, which was above discussed and 
figured (Figs. 46 and 60), and of which the protochordal plate 
is the median frontal portion. 
In the tortoise, Hmys, Mehnert (’96) gives detailed descrip- 
tions as to how this ring of tissue has in the first place the 
aspect of lateral outgrowths from the primitive streak ; later 
of crescent-shaped wings, and only finally of a ring. It 
may be here remembered that also in the embryonic shield 
of 'Tarsius the first origin of blood and blood-vessels is 
observed in the hinder part, and that we notice a similar 
wing-shaped advance in the distribution of the mesenchyme- 
producing annular zone. At the same time it should be 
borne in mind that once the primordium of the vascular 
tissue having arisen out of the entoderm its further develop- 
ment becomes independent of the region of its origin, so 
