ORIGIN OF VASCULAR TISSUES 77 



ing process to have taken place: in the anterior potential heart- 

 region the entoderm was probably in close contact with the 

 pharyngeal entoderm as illustrated in figure 7. Such a condi- 

 tion may have obtained as is illustrated in figure 7, which is a 

 hypothetical dorsal view diagram of an early stage in the devel- 

 opment of this meroplast. In figure 10 the splanchnic meso- 

 derm is closely applied anteriorly to the wall of the pharynx. 

 Anteriorly no pre-endothelial tissue is shown, just as in figure 

 7. In the posterior heart-region of figure 7 the splanchnic 

 mesoderm lies farther away from the pharyngeal wall, leaving 

 a space into which pre-endothelial tissue was proliferated. Such 

 a condition must have existed in the posterior region of the 

 cardiac coelom, otherwise the conditions in later development 

 could not be explained. As a possible transition between the 

 stage just described and the conditions actually observed in 

 figures 3, 6, 10, and 39 to 44, figure 8 is submitted. The plug 

 of vascular tissue has increased in length and has enlarged 

 somewhat anteriorly. The visceral mesoderm is giving off a 

 mesially directed fold dorsal to the pre- vascular proliferation, 

 and a similar fold ventral to it. At the same time the visceral 

 mesoderm is invaginating to receive the proliferation. What- 

 ever may have been the actual process involved, figure 9 shows 

 diagrammatically the condition found when the tissue was 

 sectioned; the plane of each section is roughly indicated by 

 arrows in this figure. The blindly ending cardiac endothelium 

 is enclosed by a blind sac of myocardium which projects an- 

 teriorly into the coelom. The conditions in the diagram are 

 greatly simplified, and no attempt was made to show the con- 

 ditions accurately. In this, as in many other instances, there 

 was a number of ridges on the inner myocardial surface in addi- 

 tion to the two principal folds which encircled the endocardial 

 tissue. These ridges are characteristic of cases in which the 

 formation of the vascular tissue is somewhat irregular or con- 

 siderably arrested. Detailed descriptions ol the actual sections 

 are given in another connection. 



To summarize the facts just considered, it seems that cardiac 

 endothelium can develop when visceral mesoderm does not 



