BY JAS. P. HILL AND C. J. MARTIN. 
67 
(2-3) endothelial tubes (fig. 5, ht. end ) which about the middle 
region of the Anlage unite into a single tube (tig. 6, ht. end.). 
Traces of a septum are, however, still present in the single tube, 
showing that it has arisen, as Rabl* has observed, by the fusion 
of at least two smaller ones. On the ventral wall of the single 
endothelial tube there is a distinct cell mass projecting into the 
cavity of the same (tig. 6) : it apparently represents the ventral 
part of the septum above mentioned. The endothelial wall is 
separated by a considerable space from the (splanchnic) meso- 
dermal wall of the heart Anlage. In the middle region of the 
Anlage the latter exists in the shape of a semi- tubular canal open 
ventrally (fig. 6, spl. ), while both in front and behind the groove 
becomes shallower and more closely applied to the somatic 
mesoderm. 
Posteriorly, at the same time the endothelial tube is reduced in 
size and is continued backwards as the Anlage of the sinus 
venosus and omphalo-meseraic vein, and with this other endothelial 
tubes unite. Near its posterior end each venous Anlage consists 
of one or two small vessels which disappear finally just anterior 
to the first somite. 
Endothelial vessels have already begun to appear in other parts 
of the embryonic region, e.g., in the mesodermal head plate and 
especially where that underlies the medullary plate (figs. 3, 4, 5 
and 6). These are not yet connected with the anterior prolonga- 
tions of , the heart Anlage, nor do they appear to contain blood 
corpuscles. It is worthy of note that the endothelial vessels may 
also occur in the somatic mesoderm, between it and the ectoderm. 
Bonnetf has also observed vessels in the somatic mesoderm in the 
sheep, but according to him they soon disappear. As already 
mentioned, a vascular area was not visible in the fresh condition, 
but sections reveal the presence of vasifactive cells and actual 
vessels in the extra-embryonic region (figs. 10, 12, 30, vas. c, b.v.). 
Both the vessels and the vasifactive cells become more numerous 
opposite the posterior end of the embryo. The vessels exist in 
* Morph. Jahrb. Bd. xv. p. 226. 
+ His's Archiv. 1889, p. 56. 
