26 VASCULOGENESIS IN THE CAT 



a perfect injection injects all the endothelium present; if more is 

 found, then the injection is a failure but the principle remains 

 unchanged. 



There is, however, no reason to assume that injection has reach- 

 ed the acme of delicacy, great as is the skill which has been de- 

 veloped in its use. It is concei\'able that some day it will be 

 possible to inject a portion of the 'anlage' before its admittedly 

 discrete vessels have fused — even a small fragment might be in- 

 jected alone, the aorta in the head say, or the umbilical vein. 

 This would then become to the injectionist the only source of the 

 whole vascular system. The uninjected parts of the anlage would 

 be demonstratedly not vascular anlages at all, but artifacts or 

 'Mayer-Lewis anlages' and no study by slide or section could 

 establish their right to serious consideration ; they would be in the 

 same status as the separate anlages of the thoracic duct, or any 

 of the mesenchymatous vesicles of Huntington and McClure, 

 and no degree of care in topographical localization, no minute- 

 ness in the gradation of reconstructed stages, no matter how fully 

 their enlargement, gradual approximation and fusion were shown, 

 would suffice to prove their participation in the formation of 

 vessels. If not injectible they can never become endothelium. 

 But the discrete vesicles and blood islands of the splanchno-pleure 

 are as an actual fact not injectible from the first and theii' struc- 

 ture is hardly known apart from sections. It would seem to be 

 a reasonable and consistent conclusion from the injectionist stand- 

 point, that the 'anlage,' in the splanchno-pleure is an artifact 

 or a 'Mayer-Lewis anlage,' and has absolutely nothing to do with 

 the vascular system,which should consistently be held to come into 

 being ex abrupto with the first moment of injectibility, for the 

 chick, at present, the stage of 20 somites. 



But some may not feel themseh-es obliged to confine their 

 knowledge of endothelium to such facts only as can be corrobo- 

 rated by injection, for there is a stage of vascular genesis admit- 

 tedly^ inaccessible to injection, and here the method of section 

 and reconstruction has acknowledged validity and further is 

 entirely consonant in its results with the data of intra vitam 

 observation. 



