56 DEVELOPMENT OF LYMPHATIC SYSTEM, FISHES 



peripheral lymphatics may exist and, if such is actually the case, 

 the question can alone be decided by a direct observation of the 

 facts. 



I have hitherto paid little attention to this particular subject. 

 I have made, however, certain observations which bear more or 

 less directly upon this point. 



In sections of the embryo one frequently meets with a con- 

 dition in which the lumen of an endothelial-lined lymphatic opens 

 directly into a mesenchymal-lined tissue space. One of the best 

 and most convincing examples of this character was demon- 

 strated by Huntington at a meeting of the American Associa- 

 tion of Anatomists in 1911 and he has kindly permitted me to 

 illustrate it here. Figure 41 is a photomicrograph of a trans- 

 verse section taken through the periaortal lymph sinus of Chely- 

 dra serpentina. According to Huntington this lymph sinus is not 

 connected with any vein and has developed in the mesenchyme 

 in an area entirely free of veins. As shown clearly in the photo- 

 micrograph, this lymph sinus (25) is lined by a continuous layer 

 of endothelium except at one end (right side of fig. 41) where 

 the endothelium ends abruptly and the lumen of the lymph sinus 

 opens directly into a typical mesenchymal-lined tissue space (26) . 



The increase in extent which this lymph sinus of Chelydra 

 was still undergoing, at the time the embryo was killed, was be- 

 ing accomplished, I believe, as the result of an addition to it of 

 this mesenchymal tissue space (26), and the continued addition 

 to the latter of similar, adjacent tissue spaces (32) would further 

 increase its extent. It is evident that this lymph sinus of 

 Chelydra with its communicating tissue space can in no sense be 

 regarded as a closed endothelial-lined tube. If it should ulti- 

 mately become one, this could be accomplished only as the result 

 of a further local in situ differentiation into endothelium of the 

 entire mesenchymal strand which forms the wall of the adjacent 

 communicating tissue space. It is quite possible that i i the 

 course of the subsequent extension of this lymph sinus, by the con- 

 tinued addition to it of mesenchymal spaces, a stage might ulti- 

 mately be reached in which the mesenchymal wall of a com- 

 municating tissue space became differentiated into endothelium 



