DEVELOPMENT OF CONNECTIVE SUBSTANCE AND BLOOD. 187 



uncertainty prevails as to whether the inner germ-layer alone, 

 or the middle, or both, are concerned in the production of the 

 fundament. 



When once the first vessels have been formed, they grow further 

 independently, and continually give rise to new lateral branches by 

 means of a kind of budding process. 



It can be observed that from the walls of vessels that are already 

 hollow, solid, slender sprouts go out, which are formed of spindle- 

 shaped cells, and by means of cross-branches join others to form a 

 network. The youngest and most delicate of these sprouts consist 

 of only a few cells arranged in a row, or indeed of only a single one, 

 which, reposing upon the endothelial tube like a knob, is drawn 

 out into a long protoplasmic filament. Into the solid sprout there 

 now projects from the already completed vessel a small evagination, 

 which gradually elongates and at the same time enlarges into a 

 tube, the wall of which is formed of the separated cells of the funda- 

 ment. The formation of blood-corpuscles no longer takes place in this 

 process, all the cells of the sprout being employed to form the wall of 

 the vessel. Since out of the vessels thus produced new sprouts 

 are formed, and so on, the fundaments of the vessels spread them- 

 selves out everywhere in the spaces between the germ-layers and 

 the organs which have by constrictions been formed from them. 



There are, moreover, two different opinions about the manner in which the 

 sprouting takes place. Are the solid vascular shoots formed exclusively by 

 growth of cells in the wall of the endothelial tube, or do neighboring con- 

 nective-tissue cells take part in their formation ? While RABL holds to the 

 proposition that new vascular endothelia always take their origin from such as 

 are already in existence, KOLLIKER, MAYER, and R(JCKERT make statements 

 which appear to prove that the endothelial vascular tubes both continue to 

 grow by themselves alone, and also to elongate through the participation of 

 the connective-tissue cells of the surrounding tissue. 



In the preceding pages we have endeavored to show in detail 

 how in Vertebrates the material of the cleavage- cells is differen- 

 tiated into the separate fundamental or primitive organs. As such 

 we must designate the outer and the inner germ-layers, the two 

 middle germ-layers, and the mesenchyme or intermediate layer. 



In order properly to estimate at once the significance and the rdle 

 of these fundamental organs, we will glance at the final result of the 

 process of development propound the question, What organs and 



