24 DEVELOPMENT OF THE SYSTEMIC LYMPHATIC VESSELS 



It is therefore quite possible that in the mammalian embryo both 

 sets of intercellular mesodermal spaces develop side by side and 

 simultaneously, although prior to the setting free of the haemo- 

 globin cells and their appearance within the lumen of the haemal 

 capillaries, there is no absolute criterion which would serve to 

 distinguish intercellular spaces as belonging definitely to either 

 the haemal or the lymphatic division of the general vascular 

 anlage. 



It hence appears to me futile to base serious conclusions in 

 regard to the genesis of vascular structure on observations made 

 on the vessels seen in the transparent tails of living anure am- 

 phibia. Phylogenetically, from the urodele standpoint, such 

 larvae are adult organisms. We all know that, once established, 

 all divisions of the vascular organization are, under the stimulus 

 of normal or abnormal growth, capable of further increase and 

 extension. The observations above referred to may offer, if 

 correctly interpreted, interesting side lights on the method of 

 vascular growth, but they cannot, in any valid sense, bear on 

 the problem of vascular genesis, either haemal or lymphatic. 



But in mammalian embryos of the proper stages, and specifi- 

 cally in embryos of the Cat between 10 and 12 mm., the first 

 systemic lymphatic anlages are clearly differentiated in the cir- 

 cumscribed areas of their first appearance, coincident with the 

 definition of the early intra-embryonic blood channels. 



Thus in the omphalo-mesenteric district, and cephalad and 

 caudad of this point, on each side of the aorta, isolated 

 intercellular mesenchymal spaces appear at this period, closely 

 applied to the walls of the neighboring venous plexuses of the 

 postcardinal and mesonephroic veins, but not connected with 

 the same. These intercellular clefts enlarge rapidly to form 

 numerous oval or round spaces, closely interwoven with the 

 venous network and later with the sympathetic anlages. The 

 cells limiting these early lymphatic anlages become, with the 

 further extension of the spaces which they line, flattened and 

 assume typical endothelial characters. 



Fig. 1 shows a transverse section of a 10 mm. cat embryo 

 (series 111, slide viii, section 4) magnified 175 diameters, cephalad 



