GENERAL CONSIDERATIONS 63 



rived from the yolk-sac angioblast. Also, the question con- 

 cerning the origin of these mesenchymal cells, whether derived 

 from entoderm, mesoderm or mesothelium (Bremer '14), does 

 not concern us here. The main point at issue is ihe establish- 

 ment of the fact that the endothelium of the intra-embryonic haemal 

 vessels is the product of a local in situ differentiation of certain 

 cells in the embryo which have not been derived from the yolk-sac 

 angioblast. 



Let us now compare these conditions of the intra-embryonic 

 blood-vascular system, as determined by sections and experi- 

 ment, with those of the embryonic lymphatic system. 



Our knowledge of the embryonic lymphatic system is gradually 

 approaching a state where, in such forms as teleosts and am- 

 phibia, it may also be possible to determine by experiment exactly 

 how the lymphatic system is formed. A thorough knowledge of 

 the lymphatic channels and the order of their appearance in the 

 normal embryo would be quite essential, however, before experi- 

 ment could be successfully applied. Since the anlagen of the 

 lymphatics do not make their appearance in the embryo under 

 normal conditions until after the veins have been established 

 and have begun to function, it is quite possible, in cases of ar- 

 rested development of the venous system, as demonstrated by 

 Stockard in Fundulus, that development might never be success- 

 fully carried to the lymphatic stage. Be this as it may, until the 

 problem has been tested by experiment, our knowledge and 

 interpretation of lymphatic development must, for the present, 

 be based upon the observation of fixed and of living material, 

 and its comparison with the known developmental stages of 

 the blood-vascular system, as observed in fixed and in living 

 material, and as verified by experimental means. If it can be 

 shown that the anlagen of the lymphatic system present exactly 

 the same conditions in fixed and in living material, as those of 

 the blood-vascular system, it is reasonable to infer that in their 

 development the lymphatic and blood-vascular systems follow 

 exactly the same genetic plan. // one were to observe that in cer- 

 tain cases intra-embryonic blood vessels were formed in the living 

 embryo by a sprouting or growth of a preexisting endothelium, 



