DEVELOPMENT OF LYMPHATICS IN ANURA el 
and the latter being in continuity with the aortic arches. In 
the reconstruction reproduced in figure 29 their topographical 
relations are clearly indicated, though this deals with a later 
stage, a 6-mm. embryo, in which the demarcation between 
jugular (v. jug. ext.) and carotid (a. car. ext.) is complete except 
anteriorly, where they are still in broad plexiform connection. 
The inception of the primary maxillary sinus takes place in 
5-mm. embryos during the period of the indifferent jugulocarotid 
plexus, just described. Its initial anlagen arise along those 
channels which are to become the external jugular veins, and at 
first many of them are in the form of short knot-like cellular 
thickenings adhering to their lining. Such alymphatic anlage is 
shown in the photomicrograph, figure 8, as a compact protu- 
berance (lym.) of the intima of the blood vessl (v. jug. ext. dex.). 
A transverse section of another sinus anlage of the same speci- 
men, but from the opposite side, is pictured in figure 9, 6. In 
longitudinal extent, it passes through seven sections (each 6 u 
thick). It is a solid cell cord or column attached to the wall 
of the vein (v. jug. ext. sin.) by its anterior end, while throughout 
the remainder of its course it lies free in the mesenchyme ventral 
and parallel to this vessel. 
Besides the adherent lymphatic anlagen, there are at this stage 
other anlagen, which, though they be similar to them in size, 
shape, and location, are not in immediate contact with the lining 
of the blood channel, and the question naturally arises: Were 
such anlagen formerly connected with the haemal endothelium, 
or did they arise independently? Observations on the succeeding 
genetic stage, as well as the investigation of the developing 
lymphatics of the trunk region, furnish evidence that points to 
the independent origin of such anlagen and besides reduces the 
significance which we would attach to the adhesion of some of 
the earliest lymphatic anlagen to the primitive blood channels. 
The theoretical aspects of this problem will be discussed after 
the steps in the development of the primary maxillary lymph 
sinus have been described. 
The lymphatic anlagen, like the endothelium of all blood 
channels, especially in the head region during early development, 
