Development of the Jugular Lymph Sacs. 259 
Subsequently, as stated in the preliminary analysis of veno- 
lymphatic development, and as will be shown in the course of the 
detailed consideration of the more advanced stages, these multiple 
early connections undergo great numerical reduction. 
This embryo, as well as series 13 (fig. 39), is also of great 
importance in illustrating a feature of veno-lymphatic develop- 
ment which, as yet, has not been carefully considered and one 
which is even more marked in certain of the succeeding stages. 
We refer to the presence of the detached and isolated veno-lym- 
phatic saes, or spaces which appear as outlying islands to the main 
divisions of the veno-lymphatic plexus and in the area in which the 
jugular lymph sacs are subsequently found. These detached sacs 
may or may not contain blood corpuscles and, in the latter case, 
they present the appearance of typicalisolated lymphatic spaces. 
The gradual separation and detachment of the main veno-lym- 
phatic plexus from the systemic veins constitutes a uniform and typt- 
cal phase in the ontogeny of the jugular lymph sacs. It, therefore, 
appears to us that the detachment of these small veno-lymphatic 
elements, in advance of the main veno-lymphatic system, may be 
explained on the ground that their detachment merely anticipates the 
main sac in attaining this stage. 
As to the subsequent fate of these detached spaces there can 
be no doubt. They enlarge, fuse with one another and with the 
main veno-lymphatic plexuses in forming the anlage of the jugu- 
lar lymph sae. 
Series 19, 9°" Embryo 
Reconstruction of right side, 
Lateral aspect, fig. 41 and 
Reconstruction of left side, 
Lateral aspect, fig. 42 
This embryo is markedly curved and shows a distinct demarca- 
tion into cephalic arch, straight precardinal segment and promon- 
tory. | It affords an instance of the developmental type described 
in the analysis (p. 211) and illustrated by fig. 9, in which the 
cephalic division of the ventral veno-lymphatic plexus is formed 
primarily from the peri-precardinal capillary plexus, involving 
