ORIGIN OF THE POSTERIOR LYMPH HEART 411 



These cells are becoming rounded, with a diameter of 7 to 8 ix. 

 Their cytoplasm is neutrophile or eosinophile and contains 

 several strongly eosinophile granules. The nucleus is slightly 

 more basophile than the cytoplasm. Eosinophile granules were 

 also observed in the cytoplasm of some of the mesenchyme cells. 

 There is usually a free space of 2 to 3 m about each differentiating 

 cell, which is not encroached upon by the surrounding mesen- 

 chyme. Lateral branches of the coccygeal veins pierce the caudal 

 muscle plate at regular intervals but the capillaries which they 

 drain are few in number. 



The 8.5 mm. embryo presents a very similar state of affairs, 

 except that the capillaries emptying into the lateral branches 

 of the coccygeal veins are somewhat more numerous, and the 

 differentiating blood cells also occur in greater numbers. As 

 may be seen from figure 1, 5, the haemal capillaries are injected 

 to the point of extravasation, but the dift'erentiating eosinophile 

 cells (7) are absolutely independent of them, nor are there any 

 eosinophile cells medial to the caudal muscle plate. 



From this stp.ge on until the embryo reaches the length of 

 10.5 or 11 mm. (fig. 2), the capillary plexus steadily increases 

 in richness and complexity, while the blood cells differentiating 

 from the mesenchyme become scarcer. The capillary plexus 

 has invaded the area formerly occupied by differentiating blood 

 cells, and blood cells in the mesenchyme have decreased until 

 only a small fraction of those present in the 8.5 mm. embryo 

 remain . 



These blood cells have, then, either degenerated and dis- 

 appeared, or have been drained off by the capillary plexus. 

 The present investigation has not been of such a character as 

 to warrant tracing the complete history of the blood cells which 

 differentiate from the mesenchyme but representatives of both 

 the red and white blood cell lines have been identified in the 

 tissue spaces. 



That these cells are drained off by the extending capillaries 

 is indicated by the fact that within five or six hours we find 

 first a practically indifferent mesenchyme, a little later a ver}^ 

 active haemopoesis taking place in it and finally a general vas- 



