BY .JAS. P. HILL AND C. J. MARTIN. 67 



(2-3) endothelial tubes (fig. 5, lit. and ) which about the middle 

 region of the Anlage unite into a single tube (fig. 6, ht. end.). 

 Traces of a septum are, however, still present in the single tube, 

 sliowing 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 tlie same (fig. 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, 

 l)ut sections reveal the presence of vasifactive cells and actual 

 vessels in the extra-embiyonic 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. .lahrb. Bd. xv. p. 226. 

 t His's Archiv. 1889, p. 56. 



