422 RANDOLPH WEST 



more spaces continue to connect with the veins, either directly 

 or by means of the delicate hollowed 'cell strands' already de- 

 scribed, and as the spaces acquire venous connection, they may 

 become filled with blood backed up from the general circulation 

 especially in injected embryos. The spaces which have not as 

 yet attained a venous connection, increase in size, several smaller 

 spaces coalescing by a breaking down of their cell boundaries 

 to form a single larger space (fig. 9, 8; fig. 8, 8; fig. 3, 8). As 

 the plasmatic pressure becomes greater, the indifferent mesen- 

 chyme cells which bounded these spaces become flattened to 

 form cells which are identical in appearance with endothelial 

 cells (fig. 9, 8). The first spaces about which endothelial cells 

 were detected were in a 13.5 mm. embryo, although the cells 

 bounding the spaces were somewhat flattened in the 11.5 mm. 

 and 12.5 mm. embryos. The fact that the cells about a single 

 isolated space may be in part endothelial and in part mesen- 

 chymal, with many intermediate stages between the two, indi- 

 cates that an in situ differentiation of endothelium from mesen- 

 chyme is taking place. 



The haemopoesis, which was described as taking place before 

 the lymphatic anlagen appear, continues, but much less rapidly 

 than formerly. We have seen that the mesenchyme lateral 

 to the caudal muscle plate was first practically indifferent and 

 non-vascular. Then came a wave of haemopoesis, followed 



Fig. 6 Chick 11 mm., Series 20, Slide 1, Row 3, Section 7. X 500. Photo- 

 micrograph ot transverse section of caudal end of the embryo. 



Fig. 7 Chick 15 mm., Series 16, Slide 2, Row 4, Section 6. X 500. Photo- 

 micrograph of transverse section of caudal end of the embryo. 



Figure 6 7, Differentiating blood cell 



6, Caudal muscle plate ^> Mesenchymal space 



9, Lateral branch of coccygeal vein , ^' Lateral branch of coccygeal vein 



12, Isolated plasmatocyst ^^> Elongated cell with pale nucleus 



13, Elongated cell with pale nucelus ^^^ distinct nucleoli 



and distinct nucleoli ^^' Lumen continuous with vein 



15, Lumen not continuous with vein 

 ^^Sure 7 j^g^ Isolated flattened cell, with pale 



3, Coccygeal vein nucleus and distinct nucleoli 



6, Caudal muscle plate 



