DEVELOPMENT OF CONNECTIVE SUBSTANCE AND BLOOD. 191 



migrates in between the primary germ-layers, and becomes blood and connec- 

 tive substance." 



According to the recent investigations of RABL, ZIEGLEB, VAN WIJHE, 

 RiiCKERT, and others, the mesenchyme is produced from various regions of 

 the middle germ-layer. A participation of the inner germ-layer in the forma- 

 tion of the blood-vessels is rendered probable. 



SUMMARY. 



1. Besides the four germ-layers, which have the form of 

 epithelial lamellae, special germs are developed in the higher 

 Vertebrates for the sustentative substances and the blood, the 

 mesenchyme-germs. The latter together make up the intermediate 

 layer. 



2. The mesenchyme-germs arise by cells detaching themselves 

 from epithelial union with the germ-layers, and penetrating as 

 migratory cells into the fissure between the four germ -layers (the 

 remnant of the original cleavage-cavity) and spreading themselves ^ul 

 in this space. 



3. Germ-layers and mesenchyme-germ (intermediate layer) ex- 

 hibit a difference in the method of their origin : the former are 

 developed by foldings of the wall of the blastula, the latter by emi- 

 gration of isolated cells from definite territories of the germ-layers. 



4. Mesenchyme-germs arise from the wall of the primitive segment, 

 from the cutis-plate, and at certain regions of the parietal and 

 visceral lamellae of the middle germ-layer. 



5. Blood-vessels are developed both in the_bodyzof the embryo 

 itself, in a manner which still remains to be accurately determined, 

 and also in the territory of the area opaca of Daeroblajtic_eggs^ 



6. The source of the cells from which the vessels~and blood of 

 the opaque area arise is at present a matter of controversy. 



7. In the formation of vessels in the opaque area the following 

 phenomena are to be regarded : 



(a) The embryonic cells of the intermediate layer arrange 

 themselves : 



First into a network of cords, and 

 Secondly into the substance-islands (Substanzinseln). 

 (6) There are developed out of the cell-cords, at the same time 

 with the secretion of the fluid portions of the blood, the 

 endothelial wall of the primitive blood-vessels and their 

 cellular contents, the blood-corpuscles (blood-islands). 

 (c) The Substanzinseln become embryonic connective substance 



