DIYILOPMW i Ul< i-: AM. BLOOD, 187 



uncertainty prevails as to win -tlr-r tho inner germ-layer alone, 

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

 fundament. 



When once the first vo-rls have been forme.!, they grow further 

 intlepei.dently. and cont inually give ri.-e to new lateral brandies ly 

 means of a kind of budding proc< 



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

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

 shaped cells, and by means of cro-l>ranrhe> join others to forma 

 network. The youngest and most delicate of these sprouts con-i-t 

 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 i.y 

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

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

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

 are already in existence, KOLLIKER, MAYER, and Ui CKKUT 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 organ-. A- 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 signifr ' the rdle 



of these fi< u<}<i mental organs, we will glance at the final result of the 

 process of development propound the question, What organs and 



