ORIGIN OF VASCULAR TISSUES 85 



endothelium can come from entoderm, one should be able to 

 find entoderm in the act of proliferating that tissue. Such pro- 

 liferation might well be expected to exhibit mitotic figures. 

 From these considerations it seemed desirable to stain sections 

 of a number of the meroplasts with iron haematoxylin. Mitotic 

 figures in the tissues were generally found to be very scarce; in 

 one case they were found to be almost incredibly abundant 

 (figs. 40 to 44). In this connection it is interesting to note that 

 in a demonstration of material at the thirty-second session of 

 the American Association of Anatomists, Sabin (61) showed that 

 mitosis in chick blastoderms proceeded by definite rhythms, the 

 actual time of mitosis being of exceedingly short duration. In 

 some of Sabin's preparations, almost every endothelial cell could 

 be seen in almost precisely the same stage of mitosis. 



As we have previously noted, Hahn stated that the basement- 

 membrane of the entoderm is never interrupted by a cellular 

 proliferation; thus he considered its proliferation of endothelial 

 tissue to be out of the question. Let us now consider certain 

 cases in which the basement-membrane undoubtedly is inter- 

 rupted. Instances of this are most often found in the heart- 

 region, such as is shown on the right side of figure 39. Internal 

 to the line of longitudinal incision, the entoderm is seen, even at 

 this low magnification, to be greatly thickened. This figure 

 will serve to locate approximately the same parts more highly 

 magnified in figure 40. Here it will be seen that certain ento- 

 derm cells are in active mitosis. The basement-membrane of 

 the entoderm is undoubtedly interrupted in two places. The 

 splanchnic mesoderm is thrown into a number of folds, none of 

 the apices of which is in contact with the tissue which the ento- 

 derm seems to be proliferating. Between mesoderm and ento- 

 derm are lightly staining strands of a coagulated plasma-like 

 fluid, stained only by the counterstain. 



Figure 41 is a section slightly anterior to the plane of figure 

 40. Slightly separated from the entoderm is a bridge of tissue 

 which can be traced back to the entodermal proliferation of fig- 

 ure 32, Traced still farther anteriorly (fig. 42) this bridge of 

 tissue is still seen to be in continuity with the entoderm. Near 



