12 VASCULOGENESIS IN THE CAT 



known to be regressive, because they are preceded by blood con- 

 taining plexuses and functional vessels and accompany their 

 resolution. Huntington 13 has noted the condition in connection 

 with the perivenous plexuses: 



Some of the elements .... develop into permanent tributaries 

 of the main veins. Others undergo a process of separation from the per- 

 manent functional channels and degenerate. In many cases their 

 blood cell contents break down and are eliminated, while their endothelial 

 lining appears to revert to the indifferent type of the embryonic meso- 

 dermal cell. 



Thus in embryos between 13.5 and 16 mm. many striking instances 

 of this reversion are to be observed. [And further] .... these 

 detached and retrograding channels .... constitute lines around 

 which the most active primary lymphatic organization of the mamma- 

 lian embryo centers. 



I am far from wishing to claim that 'sprouts' are never grow- 

 ing points, I would only instance cases where they certainly are 

 regressive, and express the doubt that their injectibility or lack 

 of it affords a proof of their origin from endothelium. The sub- 

 ject is a promising one and would repay unprejudiced observation 

 under conditions where the direction of the process might be 

 definitely ascertained in a circumscribed area and with a closely 

 graded series of foetuses. It may be that their formation in the 

 lymphatic system will prove to be a late phenomenon and inci- 

 dent to the ultimate sealing off of the lymphatics from the tissue 

 spaces, if a closed system of lymphatic capillaries actually exists. 



The question of extravasation is perhaps the most difficult of 

 the many problems the injection method has raised, and is giving 

 us a literature of as much subtilty in connection with develop- 

 ing vessels as it has done in regard to the circulation in the adult 

 spleen. The point at issue is not the permeability of the vessel 

 wall which may be taken as proved beyond controversy by the 

 facts of metabolism, and the passage of erythrocytes and leuco- 

 cytes, as well as by the results of injection. The real difficulty is 

 as to how we may accept these indisputable facts and yet main- 

 tain that when extravasations occur during injection they prove 

 a discontinuity between the lumen of the vessel and the tissue 

 space in mesenchyme or in connective tissue. 



11 1911, Memoirs of the Wistar Inst. of Anat. and Biol. No. 1., Philadelphia. 



