70 DEVELOPMENT OF PRIMITIVE BLOOD-VESSELS. 



one can readily focus through them from the endoderm to the ectoderm, and see 

 every cell of the entire zone. This is true, however, only when the endoderm is 

 not dividing. The endoderm cells divide as a whole, and during the entire phase 

 of cell division they are so opaque that it is impossible to focus through them. 

 The phase requires about an hour, and to study the vessels beneath it is necessary 

 to await until the cells become clear again. 



In the entire outer margin of the area pellucida, at the stage of 6 somites, 

 there are two plexuses. The dorsal plexus, which is the developing ccelom of this 

 area, appears to be composed of very large and flat vessels. Distinctly ventral 

 to this plexus of the coelom is another plexus, much less abundant and made up 

 of solid bands of cells, which are angioblasts. An exceedingly important point, 

 which can be determined with great distinctness in the living specimen, is that 

 the plexus of angioblasts connects by many tiny filaments with the plexus of the 

 mesoderm of the coelom, but never connects by filaments with the endoderm. In 

 sections the angioblasts of the vascular layer often touch the endoderm, but in 

 the living embryo they are always separate. The living specimens also bring 

 out very sharply the fact that the entire layer of angioblasts is distinctly ventral 

 to the plexus of the mesoderm; in other words, the term vascular layer of Pander 

 is an appropriate one, for the filaments of the angioblasts can be seen to dip down 

 from the vascular layer to the mesoderm beneath. In the flat living specimen, 

 and in sections which have been made from a specimen which was growing out 

 flat on a cover-slip, there is no intermingling of the mesoderm and the vascular 

 layer, such as is seen in Duval's plate xvi, figure 264. Such an apparent inter- 

 mingling of the two layers is due to shrinkage. In other words, the angioblasts 

 differentiate out of the mesoderm and form a new layer, which is throughout 

 ventral to the mesoderm. These two plexuses were well known to His, who 

 recognized them in their relations to the ccelom on the one hand and to the angio- 

 blasts on the other in his work published in 1868, but described them more fully 

 in the Lecithoblast und Angioblast published in 1900. His stated that the two 

 plexuses were at times very hard to analyze. 



The plexus of angioblasts is, then, distinguished first by its more ventral 

 position, and secondly by the fact that the cytoplasm of the angioblasts is slightly 

 more granular and reacts slightly more intensely to basic dyes than does the 

 mesoderm. The following criterion, however, is the one which I have found most 

 useful. In the living specimens there seems to be a sort of rhythm in cell division. 

 I have already referred to the fact that the entire endoderm may divide and 

 become so opaque that none of the cells beneath can be seen. At other times 

 the entire plexus of angioblasts over a very extensive zone will pass into the phase 

 of cell division. In this condition the cytoplasm of the plexus of angioblasts 

 becomes very highly refractile and opaque, so that it can be distinguished from 

 the plexus of the ccelom with great ease, even with low powers of the microscope. 

 The protoplasm shows this change for about an hour before the chromosomes 

 pass on to the spindle; so, in order to obtain the nuclear figures characteristic of 

 cell division, one must watch until a few areas in the plexus begin to clear and 



