424 RANDOLPH WEST 



quickly by a vascularization of the tissue and a decrease in the 

 number of blood cells in the tissue spaces. This takes the em- 

 bryo up to the 10.5 mm. stage, when the lymphatic anlagen 

 first appear. From this time onward certain mesenchyme cells 

 still seem to become rounded, break away from the surrounding 

 syncytium, and acquire eosinophile granules. In other cells the 

 cytoplasm becomes eosinophile more evenly, forming erythro- 

 cytes. These cells, which lie in the tissue spaces, for the most 

 part become included in the lymphatic anlagen, and as these 

 anlagen acquire a venous connection, reach the general circulation. 



For the first time in the 12.5 mm. embryo groups of rounded 

 strongly basophile cells, may be observed to be differentiating 

 from the endothelium near the junction of the lymphatics and 

 veins. Small clumps of rounded cells, more strongly basophile 

 than the mesenchyme or endothelial cells, are seen forming and 

 apparently splitting off from the endothelium of the lymphatics 

 (fig. 10, 16). In some of the older embryos the cytoplasm of these 

 cells accjuires an eosinophile tinge. These cells are identical with 

 the erythroblasts described by Dantschakoff (13). Finally, in the 

 13.5, 14.5 and 15 mm. embryos large aggregations of slightly baso- 

 phile cells with conspicuous eosinophile granules (fig. 11, 17) 

 are seen differentiating and splitting off from the lymphatic 

 endothelium. 



One hnal point must be noted, although it does not concern 

 the endothelium of the lymphatic plexus. In the 14.5 mm. 

 embryo strands of three or four myoblasts appear in the now 



Fig. 8 Chick 14 mm., Series 17, Slide 2, Row 1, Section 6. X 300. Photo- 

 micrograph of transyerse section of caudal end of the embryo. 



Fig. 9 Chick 15 mm., Series 16, Slide 2, Row 4, Section 6. X 600. Photo- 

 micrograph of transverse section of caudal end of the embryo. 



Figure 8 Figure 9 



3, Coccygeal vein 3, Coccygeal vein 



5, Haemal capillaries 6, Caudal muscle plate 



6, Caudal muscle plate 8, Isolated space, bounding cells bc- 

 8, Mesenchymal space coming flattened 



10, Lymphatic connected with veins 9, Lateral branch of coccygeal vein 



10, Lymphatic connected with vein 



