42 VASCULOGENESIS IN THE CAT 



In figures 8 and 9 are shown sections of a vesicle in relation to 

 the second nephrotome of the four somite embryo, which it will 

 be seen has some superficial resemblance to a pronephric anlage. 

 There is also a smaller one opposite the incipient fifth somite 

 and a solid mass opposite the first somite. In the eight somite 

 embryo, there is a similar sac-like anlage at the interval between 

 the first and second somites; elsewhere along the nephrotomes 

 are scattered mesenchyme cells. The formation of clefts has also 

 been initiated in the lateral portion of the mesenchyme at several 

 points. 



The comparison between these embryos does not suffice to 

 give a local individuality to the several endothelial anlages, it 

 merely shows the inception of the latter tissue diffusely from 

 mesenchyme. In older embryos the condition is found repeated- 

 ly and appears to be an intermediate stage between mesenchyme 

 and endothelium, as it certainly is intermediate in form between 

 the mesenchyme mass and the endothelial sacs; these cells of 

 intermediate form are here designated vasofactive. 



In the umbilical line, the formation of endothelium is acceler- 

 ated cephalad; almost none is found caudal of the fifth somite in 

 the ten somite embryo CColumbia Collection No. 532). Its 

 distribution is shown in figure 10. There is abundant mesen- 

 chyme in the hypectodermal space as far caudad as the third 

 somite. In its midst, but tending to a deep position near its 

 junction with the compact mesoderm, are groups of cells with 

 more abundant protoplasm which stains a little faintly. They 

 form small masses, in intimate syncytial connection with the. 

 ordinary mesenchyme. In their interior are frequently small 

 clefts. In some, the cavity is larger and the enclosing cells are flat- 

 tened, eventually becoming thin and endothelioid (fig. 11). These 

 last we may call angiocysts, using Bremer's term. The presence* 

 of several minute clefts or vacuoles in a single mass of vasofac- 

 tive cells, suggests their formation by the running together of 

 these small cavities. In figure 10 the ordinary mesenchyme is 

 omitted, the clefts and cavities are shown in red, the protoplasm 

 of their bounding cells in black. Opposite the fourth and fifth 

 somites are scattered cells and cell-groups of a similar appearance 

 in which cavities have not yet appeared. These are readily dis- 



