414 RANDOLPH WEST 



1. Histogenesis 



In the embryo of 10.5 mm. (about 4 days and 22 hours) we 

 observe two new phenomena; the formation of spaces bounded 

 by mesenchymal cells which eventually become flattened to form 

 an endothelium, and the appearance of certain strands of flattened 

 cells in the mesenchyme. Haemopoesis continues to take 

 place in the mesenchyme and also from endothelial cells of the 

 lymphatic walls as soon as these are formed. 



Throughout the younger stages until the embryo has reached 

 the length of 10.5 mm. the mesenchyme lateral to the caudal 

 muscle plate is of a uniform degree of compactness equal to 

 that of the mesenchyme medial to the muscle plates. The 10.5 

 mm. embryo, however, shows a slight, but distinct loosening 

 of the mesenchyme just lateral to the muscle plate, between the 

 points of penetration of the lateral branches of coccygeal veins 

 and at certain points the loosening of the tissue is more marked, 

 giving rise to small mesenchymal spaces. The spaces still 

 bounded by mesenchyme are more numerous in the 11 mm. 

 embryo (fig. 2, 8) and some differentiating blood cells have be- 

 come included in them (fig. 5, 7). Certain of the spaces nearest 

 the veins have acquired a venous connection at this stage and 

 in the injected embryos appear as small knob like processes 

 (fig. 5, 10) of a larger caliber than the veins with which they 

 connect, filled with blood cells, and lined by endothelium. These 

 knobs correspond in shape to the mesenchymal spaces men- 

 tioned. It is to be expected that when a space connects with a 

 vein and is subjected to the pressure and friction of the general 

 circulation, that the cells bounding it will tend to become flat- 

 tened. And the fact that in later stages, when the still isolated 

 spaces become larger and are under a greater plasmatic pressure 



Fig. 2 Chick 11 mm.. Series 20, Slide 1, Row 4, Section 3. X 200. Photo- 

 micrograph of transverse section of the caudal end of the embryo. 



1, Notochord 5, Haemal capillaries 



2, Neural tube 6, Caudal muscle plate 

 S, Coccygeal vein 8, Mesenchymal space 



4, Coccygeal artery 9, Lateral branch of coccygeal vein 



