EARLIEST BLOOD-VESSELS IN MAN 455 



I have mentioned the ''unhned spaces" which seem to form a 

 part of the vascular net, and wish now to describe them and the 

 other portions of the net in some histological detail. In the Herzog 

 embryo (fig. 4) a long funnel-shaped diverticulum of the coelom, 

 bounded by a definite mesothelial layer, is seen to expand at 'its 

 distal end (a) . The two walls approach each other in the middle 

 portion of the diverticulum, and are either in contact or definitely 

 fused, thus cutting off a distal cavity. If the fusion of the walls 

 had continued further toward the main coelom, the distal cavity 

 would appear as an irregular space, deep in the mesenchymal core 

 of the body-stalk, but connected with the mesothelial surface 

 layer b}^ a cord of mesothelial cells. Moreover, the cavity would 

 be a portion of the coelom, and its bounding walls would be also 

 inesothelium. If we turn now to the Grosser embryo, we see 

 another cavity (fig. 6, a) ; the walls are histologically similar to the 

 mesothelium {^nes.) covering the body-stalk at this point, and are 

 connected with the surface layer (at the top of the drawing) by a 

 cord of the same type of cells. The funnel-shaped mouth of the 

 original diverticulum is still clearly seen. Both on the coelomic 

 surface and bounding the cavity, the protoplasm of this layer forms 

 a narrow but definite sheet, and is connected by numerous proc- 

 esses with the underlying mesenchyma. The nuclei are broader 

 than the sheet of protoplasm, and project now toward the cavity, 

 now toward the mesenchyma. The mesothelium of the surface, 

 the mesothelial cord, the walls of the enclosed cavity, and the 

 surrounding mesenchyma all form a syncytium, as no cell walls are 

 present. The shape of this cavity as traced through the sections 

 and the fact that in other sections there are other mesothelial 

 cords connecting it with the surface make it probable that several 

 smaller cavities have coalesced to form this one. A much smaller 

 cavity is seen in the same drawing at b, near the surface. 



Another similar cavity, also from the Grosser embryo (fig. 7), 

 shows the same characteristics as the last described, except that 

 the funnel-shaped opening into the coelom has apparently become 

 obliterated by a more complete fusion of its walls, so that the 

 mesothelial cord now springs from the under side of a smooth 



