DEVELOPMENT LYMPHATIC SYSTEM IN AMNIOTES 273 



than is the case in amniote embryos. A resume of the results 

 of his investigations has appeared in the Proceedings of the 

 Seventeenth International Medical Congress. 



In mammalian and reptilian embryos my own researches of 

 the past three years have convinced me of the existence of pre- 

 cisely the same physiological and morphological conditions in 

 the area of the jugular lymph sacs and some of their tributaries. 



I can, perhaps, best illustrate these conditions, with reference 

 to the previous publications on this subject, by the following 

 series of purely schematic diagrams, whose details, however, are 

 amply supported by the study of serial sections and reconstruc- 

 tions. In the very early stages in cat embryos the precardinal 

 veins (1) and their main dorso-medial somatic tributaries (2) ap- 

 pear clearly defined in the uniform mesenchymal field (3) (fig. 4).* 



Subsequently (fig, 5) the mesenchymal area dorso-lateral to 

 the main precardinal line shows numerous small collections of 

 developing blood cells (4). These cell-masses are clearly difTer- 

 entiated from the free blood cells circulating in the veins by their 

 characteristic reaction to the Mann stain and by the fact that 

 they gradually fade off on their periphery into the indifferent 

 mesenchyme. They are not surrounded by an endothelial en- 

 velope, but appear to be imbedded directly in the mesenchymal 

 syncytium from which they arose. They are as a matter of fact 

 nothing' else than intraembryonic representatives of the early 

 vascular strands of the extraembryonic area vasculosa. This 

 stage is of very brief duration and is followed by a transitional 

 or prehaemophoric stage (fig. 6) in which a number of inter- 

 cellular clefts (5) appear in the mesenchyme, lined by endo- 

 thelium and in close apposition to the blood-islands. 



This phase practically offers a replica of the pictures obtained 

 by Whipple and McWTiorter in their study of the developing 

 vascular system in the blastoderm of the chick in vitro (27), 

 and by Schulte in the publication on the early stages of vasculo- 

 genesis in the cat, with especial reference to the mesenchvinal 

 theory of endothelium (Wistar Institute Memoirs, no. 3, 1914). 



In the succeeding stage (fig. 7) the isolated intercellular mes- 

 enchymal spaces have united into an intricate plexus (8) whose 



