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 EABL, ZIEGLER, 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 out 

 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 body of the embryo 

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

 and also in the territory of the area opaca of meroblastic 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). 



(b) 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. 



