452 ON THE ORIGIN AND EARLY DEVELOPMENT 



of veno-lymphatic and pre-lymphatic vessels as preceding the appearance of the 

 jugular sac in cat embryos, and Miller (1912) figured a similar discontinuous pre- 

 lymphatic plexus in the region of the jugular lymph-sac of chicks. 



In 1909 Mierzejewski studied the superficial lymphatics of chick embryos by 

 the injection method of Hoyer, and showed that there is a plexus of capillaries 

 present in the region of the posterior lymph-heart in chicks of 5 days, 24 hours 

 earlier than the first appearance of the lymph-heart rudiment, as described by Sala. 

 Mierzejewski considered this to be a lymphatic plexus which had grown out from 

 the coccygeal veins. 



In 1911 we began our studies of this region in chick embryos and portions of 

 these have been published from time to time. We observed that the lymph-heart 

 begins to pulsate in chicks of 6 to 6| days, and that the early contractions occur only 

 at the time of the periodic movements of the embryo. When granules of India ink 

 were injected into this translucent, pulsating area, a rich plexus of lymphatic capil- 

 laries was revealed, thus showing that the lymph-heart of the chick is a functioning 

 heart before it is a sac (Clark and Clark, 1912, 1914). Similarly, a continuous 

 plexus of lymphatic capillaries was injected in the jugular region at a stage in which 

 Miller had found the discontinuous "pre-lymphatic" anlage (E. L. Clark, 1912). 



While studying this posterior lymph-heart region in living chicks of 5 to 6 

 days, a stage before a beating lymph-heart is present, we found in this and adjacent 

 areas a richly anastomosing plexus of capillaries, independent of the surrounding 

 blood-capillaries and easily distinguishable from them, chiefly by means of the 

 stagnant blood which they normally contained. Preliminary reports of studies of 

 these early lymphatics, made on living chicks, have been published (Clark and Clark, 

 1912). The discovery that early lymphatics contain stagnant blood enabled us to 

 directly inject lymphatic capillaries with much greater facility and precision than 

 before, and by this method to study these early vessels and their relation to the 

 blood- vascular system (E. L. Clark, 19126). A preliminary report of microscopic 

 studies of these early lymphatics has also been published (E. R. Clark, 1914), in 

 which it is stated that the early lymphatic endothlium possesses definite morpho- 

 logical characteristics which distinguish it from the mesenchyme cells. 



In later studies made on pig embryos Sabin (1912, 1913) reported the finding 

 of lymph-capillaries, filled with stagnant blood, which were connected with the 

 jugular vein and with the abdominal veins in stages before sacs are present in these 

 regions. Also, W T est (1915) published a study of the posterior lymph-heart region 

 in the chick, in cross-sections of which he found a continuous plexus of lymphatic 

 capillaries before the formation of a lymph-heart. 



These various studies apparently demonstrate the incorrectness of the view 

 that lymph-sacs and lymph-hearts are the primarj^ structures of the lymphatic 

 system, and also that they are derived from veno-lymphatics which at one time 

 functioned as blood-vessels containing circulating blood. They point instead to 

 the existence of a continuous plexus of lymphatic capillaries which subsequently 

 becomes converted into lymph-hearts and lymph-sacs. 



