^2 DEVELOPMENT OF CONNECTIVE TISSUE. 



the protoplasm forms a thin layer over the entire sm-face of tlie 

 tube. It then stiffens, forming a membrane which differs in 

 no respect from that of the parent-vessel, with which, indeed, 

 it is uninterruptedly continuous. Should connective-tissue cor- 

 puscles have taken part in the construction of the new vessel, 

 thej take their place, after the metamorphosis is completed, among 

 the constituent elements of the endothelial tube. 



Secondarij vascular isation is almost exclusively confined to 

 the domain of pathology ; it may be regarded as a modification 

 of the tertiary mode in so far as it also is associated with a 

 gradual dilatation of the endothelial tube. It differs from it, 

 however, in the greater prominence assumed by the apposition 

 of new elementary parts to the wall of the vessel. The course 

 of the future vessel is mapped out b}' a cord of spindle-shaped 

 cells, arranged in parallel rows ; these cells undergoing direct 

 <3onversion into endothelial elements when the axial lumen comes 

 to be thrown open. The many varieties of this secondar}- mode 

 of vascularisation will be more fully examined under the head 

 of Inflammation and several species of tumours. 



§ 72. The second constituent of the intermediate apparatus 

 of nutrition is the connective tissue. From the embryological 

 point of view, the term '' connective tissue" embraces that residual 

 portion of embryonic tissue which is left between the blood- 

 A^essels on the one hand, and the functionally active tissues on 

 the other. The parenchymatous islets of the middle layer of 

 the blastoderm are either used up or pushed aside during the 

 development of the organs ; and in proportion as they undergo 

 one or other of these alternatives, we find the organs provided 

 with a larcrer or smaller amount of connective tissue. In some 

 organs hardly any connective tissue can be shown to exist, as f.g. 

 in the kidney and the testicle ; the lobules of the liver appear to 

 be exclusively made up of capillary vessels and secreting cells. 

 Yet it is possible to discover with absolute certainty small 

 quantities of unformed connective tissue both in the testicle and 

 in the renal parenchyma ; and where even this minimum is lacking, 

 as in the hepatic lobules, the connective tissue is represented by 

 the capillary walls themselves. For the cells of the endothelial 

 tube are equivalent to those of the connective tissue ; indeed, we 

 have already seen that the latter are capable of directly replacing 

 the former in the tertiary mode of vascularisation. Moreover^ 



