478 ON THE ORIGIN AND EARLY DEVELOPMENT 



seemed to be some evidence in this specimen that these lymphatic vessels (clearly 

 recently formed structures) were in process of differentiation from blood-vessels; 

 but, as was the case in the study of the differentiation of the lymphatics of the 

 lymph-heart region, we can state only that such a theory can not be settled definitely 

 from a study of fixed material. 



The convincing points obtained from the study of sections of an embryo 

 operated on in this manner are : 



(1) That undoubted lymphatics are present in a region which had been 

 effectively isolated from all known points of origin for its lymphatics. 



(2) That these lymphatics are abnormal in appearance for this stage, and, in 

 comparison with those of the unoperated side, resemble primitive lymphatics to 

 such a degree that it would seem highly probable that they were undergoing the 

 early development described for the first lymphatics of the posterior lymph-heart 

 region. The greater size of some of the vessels of this plexus is probably attribu- 

 table to the greater looseness of the surrounding tissue and to the consequently 

 greater expansion of these newly-formed vessels, for it will be remembered that the 

 earliest lymphatics normally differentiate in a comparatively dense tissue. 



Thus, in operation 4 it was shown that lymphatics will develop in situ in a 

 region that has been completely isolated from the points of origin from which, 

 presumably, it would normally receive its lymphatic supply. These vessels are not 

 normal for this stage of development and do not resemble those of the unoperated 

 side. From the fact that, unlike the normally appearing plexus found in embryos 

 after operation 3, no plexus of vessels was injected, and also from the appearance 

 of these lymphatics in sections, they appear to be primitive vessels which develop 

 in situ in a manner comparable with that described for the earliest lymphatics of 

 the posterior lymph-heart region. The development of such lymphatics is evidently 

 delayed in this case, since only these primitive vessels are present at a stage in which 

 the opposite side possesses an abundant lymphatic plexus. 



SUMMARY OF RESULTS OBTAINED FROM OPERATIONS ON CHICK EMBRYOS. 



The experiments just recorded have given the following answers to questions 

 of the points of origin for the superficial lymphatics of chick embryos: 



1. Regions of origin for the superficial lymphatics of chicks are not limited to 

 those areas in which venous connections are retained during embryonic life i. e., 

 to the regions of the so-called primary lymph-sacs which, in the chick, are beneath 

 the shoulder and are associated with the veins near the duct of Cuvier, and the 

 tail region associated with the first five coccygeal veins. 



2. There are at least four such regions of origin for the superficial lymphatics 

 of chick embryos: the two mentioned above, where the venous connections are 

 retained, and two others, one in the side region, probably that portion just anterior 

 to the leg, and another on the posterior part of the pelvis. In the latter two no 

 venous connections can be demonstrated after the early lymphatic plexus has 

 become continuous over the surface of the body (operations 2 and 3). 



