THE LATERAL PHARYNGEAL LYMPHATIC 35 



ever, the marked variations which occur in the rate at which 

 these anterior independent vesicles may develop, and may be- 

 come concrescent with one another, not only among different 

 embryos of the same age, but even upon opposite sides of the same 

 embryo. Such variations are of frequent occurrence and an ex- 

 ample is shown in figure 5, in which it is seen that a single inde- 

 pendent anlage (3') of the anterior portion of the lateral pharyn- 

 geal lymphatic has made its appearance only upon one side of 

 the embryo. This question will be dealt with further on in 

 connection with another topic (Section XI, page 50). 



I will not attempt to illustrate all of the independent anlagen 

 of the anterior portion of the lateral pharyngeal lymphatic which 

 are shown in the figures of the reconstructions. The examina- 

 tion of a few typical sections taken from different embryos, at 

 approximately corresponding levels, will suffice to show that these 

 anlagen make their appearance in a uniform manner in a graded 

 series of embryos, and that they therefore cannot be regarded as 

 artifacts. 



The earliest of these anlagen to make their appearance arise 

 about opposite the origin of the first efferent aortic arches (16). 

 An examination of the reconstructions shows such anlagen (3') 

 to be present on the right side of the sixteen-day steelhead (fig. 

 5, left side) and on both sides of the embryo in all of the subse- 

 quent stages, as illustrated by figures 6 to 9, inclusive. 



Transverse sections of a sixteen (fig. 27) and seventeen-day 

 steelhead (fig. 28) and of a twenty-day rainbow trout (fig. 29) 

 taken near the dorsal end of the first efferent aortic arches (16), 

 pass through these earliest independent anlagen (3'). When 

 compared with figure 30, which is a section of a twenty-two-day 

 rainbow trout taken at a corresponding level, it is seen that the 

 continuous lateral pharyngeal lymphatic (3) in the twenty-two- 

 day embryo occupies the same relative position in the walls of 

 the pharynx as its independent anlagen of earlier stages. 



Transverse sections taken through a seventeen-day steelhead 

 (fig. 31) and a twenty-day rainbow trout (fig. 32) in the region 

 of the hyoidean arteries (15), also pass through independent 

 anlagen (3') which correspond in their position to that occupied 



