OF THE LYMPHATIC SYSTEM OF THE CHICK. 459 



utes, at the expiration of which more blood enters the lymphatics and the anterior 

 (lower) portions of the plexus become much redder and more distended. 



(2) Again, the amount of blood in the early lymphatic plexus can be increased 

 noticeably and quickly by interfering with the blood circulation. When the heart 

 becomes embarrassed from any cause (such as the addition of strong chloretone or 

 too high temperature of the warm chamber) there occurs a back pulsation in the 

 veins, which can be observed to the best advantage in the large vessels of the allan- 

 tois and yolk-sac. Within a few seconds after the beginning of such a circulatory 

 disturbance the superficial lymphatics become markedly redder and more con- 

 gested, owing to the increased amount of blood which enters them as a result of this 

 increased venous pressure. 



(3) Further evidence for this backing up of the blood from the veins was found 

 in our studies (1915) of the beginning lymph flow in chicks of 6 to 6| days. In these 

 we traced the steps in the process by which the blood is washed out of the lym- 

 phatics. In the stages of its first pulsation, while the flow is relatively feeble, the 

 lymph-heart was observed to fill up with blood in the interval between beats. 

 These observations also showed that there is a stage in the development of the 

 superficial lymphatics in which the pressure in the lymphatics overcomes that 

 of the veins. 



(4) In chicks of 7 days, a stage in which there is normally no stagnant blood 

 present in the lymphatics, it was found that the flow of lymph is more rapid and the 

 lymph-heart, contractions are stronger. If the action of the lymph-heart is para- 

 lyzed by chloretone, a drug which also increases slightly the back pressure in the 

 veins, the lymph-heart and adjoining vessels soon fill up with stagnant blood and 

 become visible once more in the living embryo. 



(5) The pictures obtained from sections of chicks of 5 days also bear out this 

 point, for the newer portions of the plexus (those portions which do not possess a 

 continuous lumen) are quite empty of blood-cells, and hence invisible in the living; 

 and the blood is present only in the portion next to the veins where the lumen is 

 continuous. 



All these observations, therefore, show that the stagnant blood present in the 

 lymphatic plexus of chicks of 5 to 6 days has backed up from the veins. Its presence 

 is merely a transitory pressure phenomenon. 



Some writers have claimed that chick lymphatics have a "hemophoric" func- 

 tion, meaning that they carry newly-formed blood-cells from the tissues to the 

 blood-vessels (Miller, 1913; West, 1915). That lymph-capillaries have the power 

 of picking up blood-cells that have become extruded into the tissues was shown by 

 one of the authors in 1909. In sections of some of the earlier chicks of this series 

 (4 to 5 days) peculiar, large cells were found to be present in the mesenchyme, 

 which might be interpreted as blood-forming cells or as large phagocytes; but for 

 the stages in which the blood-cells are present in such quantity as to render a large 

 plexus visible under the binocular microscope or even to the naked eye, all the 

 evidence points to the backing-up of this blood from the veins as the true inter- 

 pretation. 



