420 RANDOLPH WEST 



exist between them and the typical mesenchymal cells shows 

 clearly that an in situ differentiation of endothelial cells takes 

 place (fig. 6, 13, and fig. 7, 19). The cells so formed may then 

 bound isolated cysts filled with plasma (fig. 6, 12) which some- 

 times enclose a differentiating blood cell. These plasmatocysts 

 then proceed to grow together connecting up with one another 

 and with the veins, and it is probable that they form in some 

 instances a connecting link between the veins and the large 

 lacunae in the mesenchyme. The early appearance of the 

 blood-filled lymphatic plexus connected with the veins in the 

 living chick, which E. R. and E. L. Clark (5) describe as follows, 

 lends weight to such an interpretation of the facts : 



The first evidence of lymphatics in the tail region of the living chick 

 is the appearance of separate knobs filled with stagnant blood just 

 lateral to the coccygeal veins. Soon after these knobs appear similar 

 ones develop about them which have fne connections with them. 

 * * * * Their injection shows discreet tiny clusters, somewhat 

 like bunches of grapes (p. 254). 



Figure 6, a section of the caudal region of an 11 mm. embryo, 

 shows an isolated plasmatocyst {12). This section and the adja- 

 cent sections were studied with the greatest care under the oil 

 immersion lense, and the two elongated cells (IS) with pale nuclei 

 and distinct nucleoli bounding the cyst were not in connection 

 with any other endothelium. 



Figure 7, a section of the caudal region of a 15 mm. embryo, 

 shows a structure which some might describe as a venous sprout. 

 The injection mass has entered the lumen for a short distance in 

 large amounts. Then the lumen becomes somewhat constricted, 

 and beyond that point only occasional ink granules can be found. 

 Finally the lumen terminates and a long flat cell (IS) follows in 

 which two distinct nucleoli are seen, beyond which is a space (15) 

 bounded by a delicate strand of cytoplasm on either side. This 

 space contains a differentiating red blood cell (7). The adja- 

 cent sections have also been examined with great care, and the 

 one directly preceding shows one rather elongated flattened cell 

 with a pale nucleus forming the floor and probably the end of the 

 plasmatocyst containing the blood cell just described. Several 



\ 



