204 George S. Huntington and Charles F. W. McClure. 
onic pathways exhibit a typical and characteristic picture, best 
defined by the term ‘‘fenestration.”’ The main lines of venous 
drainage have already become fairly defined out of the antecedent 
primary capillary network from which they arise. In the further 
growth of the embryonic tissues secondary plexiform capillaries 
surround the main vessel and become connected with the same. 
By condensation of this plexus and enlargement of the interreticu- 
lar spaces the principal embryonic veins appear in this develop- 
mental phase as channels perforated by larger spaces or “fenestrae” 
within the general confines of the main trunk. In a subsequent 
stage confluence of these ‘‘fenestree’’ results in the more or less 
complete separation of the primary vein into two parallel second- 
ary channels, which appears to be a uniform principle in the devel- 
opment of parallel venous trunks. Or, as in the case of the jugular 
lymph sae, further extension of this same process may result in 
separating from the main venous channel elements which unite to 
form a closed sae entirely distinet from the vein from which it 
arose. 
* means, therefore, in the sense in 
which it is employed in this paper, one of the last stages in the 
definite crystallization of the venous system out of an indefinite 
antecedent plexiform condition, and the determination of an 
important element in lymphatic organization, closely associated 
with the embryonic venous system. It does not mean the forma- 
tion of lacuns or spaces in veins already fully established, but is 
used by us as a convenient and short term to define the conditions 
actually and frequently encountered in reconstructions of the 
veno-lymphatie period. Thus the description of a portion of the 
arly venous area ‘‘separating by fenestration’? from the remainder 
to form a discrete and separate element is to be interpreted as 
indicating in a few words the more complicated processes just 
outlined. Such ‘‘fenestral separation’? may result in the estab- 
lishment of a secondary vein paralleling the course of the main 
channel, as in case of the posteardinal and primitive ulnar veins, 
or, specifically, in the development of the jugular lymph saes_ the 
areas involved ‘“‘separate by fenestration”’ from the vein proper to 
constitute the anlages of that portion of the lymphatic system 
The term ‘fenestration’ 
