chap.x.] THE HEART AND BLOOD-VESSELS. 





verted into a new capillary branch. Such growing 

 capillaries are capable of contraction (Strieker). 



All blood-vessels, arteries, veins, and capillaries, 

 in their early stages, both in embryonal and adult life, 

 are of the nature of minute tubes, the wall of 

 which consists of a simple endothelial membrane. 

 In the case of the vessel becoming an artery or vein, 

 cells are added to the outside of the endothelium, 

 thus forming the elastic, muscular, and fibrous con- 

 nective tissue elements of the wall. 



109. In the first stage, both in the embryo and 

 in the adult, the -y 



vessel is repre- 

 sented by a solid 

 nucleated proto- 

 plasmic cell, elon- 

 gated or spindle- 

 shaped or branch- 

 ed. Such a cell 

 may be an isolated 

 cell of the connec- 

 tive tissue inde- 

 pendent of any 

 pre -existing vessel , 

 or it may be a 

 solid protoplasmic 

 outgrowth of the 

 endothelial wall of 

 an existing capil- 

 lary vessel (Fig. Fig< 5 i._Developing Capillary Blood-vessels 

 5 1 ). In both cases *>m the Tail of Tadpole, 



it VPf>nmP linl * Capillary vein with clumps of pigment in the wall ; 

 ib ueuuuiCB iiui- ffl> nucleated protoplasmic sprout ; l, solid anasto- 

 lowed OUt by a ^' between two neighbouring capillaries. 



process of vacuola- 



tion; isolated vacuoles appear at first, but they 

 gradually become confluent, and thus a young vessel is 

 formed, at first very irregular in outline, but gradually 



