ORIGIN OF VASCULAR TISSUES 75 



longitudinal fold ventral to it, so that in section there appears 

 a funnel-shaped reflection of the splanchnic mesoderm towards 

 the dorsal surface of the endothelial tube, and a similar one 

 ventral to it. In case there be more than one endothelial tube, 

 one 'funnel' may project between two tubes (figs. 28 and 29). 

 It is rare indeed to find the apex of such a 'funnel' abutting 

 against or pointing towards the middle convexity of the surface 

 of such a tube. These mesodermal folds have a tendency to 

 project into a plane mesial to the endothelial tube to meet each 

 other, thus completely engulfing the endothelial tube (fig. 3). 

 The internal dorso-mesial element of the upper 'funnel' may 

 fuse with the internal ventro-mesial element of the lower 'funnel;' 

 at the same time the external ventro-lateral element of the 

 dorsal 'funnel' may fuse with the external dorso-lateral element 

 of the ventral 'funnel.' When these fusions have taken place 

 (fig. 43) the process may be carried still further. The layer of 

 mesoderm engulfing the tube may separate from the remaining 

 mesoderm at the riphe of fusion of the apices of the two mesial 

 folds or 'funnels,' so that the endothelial tube is now (fig. 4) 

 completely surrounded by a cylinder of mesoderm which is in 

 every way comparable to a myocardium except that there is 

 neither a dorsal nor a ventral mesentery. What corresponds 

 to one lateral half of the normal dorsal and ventral mesenteries 

 now lies mesial to the unilateral heart as a sheet of mesoderm 

 lying parallel to and closely against the entoderm; anteriorly 

 and posteriorly this mesial surface of the unilateral cardiac tube 

 necessarily joins this layer of mesoderm if the cardiac tube 

 again re-enters the body-axis (fig. 4) ; also the remaining surface 

 of the myocardium is reflected back as parietal mesoderm. 



The preceding explanation applies to the condition in which 

 the unilateral myocardial tube 'leaves' the pericardial cavity and 

 joins again the axial portion of the embryo. I have repeatedly 

 found such conditions. Let us now consider a case in which 

 the cardiac tube ends blindly in a unilateral pericardial cavity. 

 Experiment T^^pe II, No. 14 illustrates such a condition (figs. 

 3, 5, 9, 10, 39, 41, 42, and 43). The results obtained by this 

 experiment can perhaps be explained by imagining the follow- 



