DEVELOPMENT OF LYMPHATICS IN ANURA 83 
thelium has become specialized, that is, has acquired the attri- 
butes of the typical flattened lining cells. The second method 
is seen in the formation of the jugular lymph sac of mammals, 
and in that of the anterior lymph hearts and the jugular lymph 
ducts of the toad. The development of the latter structures 
will be described in following sections of this paper, but it may 
be stated here that they arise from vessels which, at first, are 
freely confluent with the embryonic blood vessels and function 
as such, but later separate and become an integral part of the 
lymphatic channel system. The third method, the formation of 
a lymph vessel by the fusion of mesenchymal spaces, somewhat 
like the origin of the earliest vascular channels, is illustrated by 
the development of a considerable portion, at least, of the thoracic 
duct and other large lymph vessels in mammals, birds, reptiles, 
and fishes, as portrayed in numerous papers that have appeared 
within the last decade. It was therefore not surprising to dis- 
cover this method active also, perhaps solely, in the formation of 
the large lymph ducts in Anura which arise later than the anterior 
lymph hearts and primary maxillary sinus, at a time when the 
blood circulatory system of the embryo had become better 
organized and its components more specialized. 
THE DEVELOPMENT OF THE JUGULAR LYMPHATIC 
Hoyer describes the development of the jugular lymphatic 
(cephalic duct) in frog embryos as a centrifugal outgrowth of the 
anterior lymph heart, but the writer’s observations show that in 
toad embryos, at least, its origin is not so simple. 
The jugular lymphaties (fig. 28, lym. jug.), one on each side, 
develop at the same time as the anterior lymph hearts and in 
the same general region so that they might be discussed together, 
though for systematic reasons they will be treated separately. 
Figures 31 to 35, inclusive, which illustrate reconstructions of the 
important structures in the territory of the left pronephros in 
several consecutive stages, furnish a clear idea of the salient and 
progressive events that occur. Besides the vascular channels 
which are directly and indirectly concerned in the formation of 
the lymphatics, other organs, such as the pronephros, spinal 
