58 DEVELOPMENT OF LYMPHATIC SYSTEM, FISHES 



Since the lymphatics merely represent a component part of a 

 general vascular system, to which the haemal vessels also belong, 

 the probability at least, is that, in the genesis of their endothelium, 

 and in the establishment of a continuous system of vessels, the 

 lymphatic and haemal vessels should follow a common genetic 

 plan. Let us consider, in the light of the more recent investi- 

 gations in the field of the vascular system, what this plan may be. 



It is not necessary to review in detail the investigations of 

 those who have hitherto maintained that the endothelium of 

 the intra-embryonic blood- vascular system possesses a local 

 origin. For an historical and critical account of this subject it 

 is sufficient to refer to the investigations of Riickert and Mol- 

 lier ('08), Felix ('97) and to the more recent one by Schulte 

 ('14) on the development of the intra-embryonic blood- vascular 

 system of mammals. 



It should be clearly borne in mind that, until quite recently, 

 the investigations which have dealt with the origin of intra- 

 embryonic vascular endothelium have not been experimental in 

 character, but have been based largely upon a study of fixed 

 material in which, however, a local and discontinuous origin of 

 blood-vascular anlagen has been observed. 



Let us now see how the view that the endothelium of the intra- 

 embryonic blood-vascular system develops in situ, and does 

 not grow into the embryo from the yolk-sac, is borne out by 

 experiment. 



Two types of experiment have thus far been made to determine 

 this question: (1) The partial separation of the embryo from the 

 yolk-sac, or, the complete separation and isolation of a portion 

 of the embryo from the rest of the embryo and from the yolk-sac, 

 at a time prior to the possible invasion of the embryonic axis 

 by the yolk-sac angioblast; (2) by observing the effects pro- 

 duced on the developing blood-vascular system in embryos 

 which have been allowed to develop under the influence of anes- 

 thetics or other chemical agents. 



The experimental investigations of Hahn ('09), and Miller and 

 McWhorter ('14) have shown, by effecting a separation on one 



