01 OOmm nvE SUBSTAN- i: \ND BLOOD. I'.H 



IB t.rtwrrn tin; primary germ-layers, and becomes blood and c 



nice." 



Dg to the recent invot illations of liABL, Xn:;i.i;i:. v\\ \Vi.inr. 



KI HI I:T. ami otiu-is, tin- met* i t'rmu vftrioiu regi 



.-Mir Lrrrm-lay.-r. A \ art i<-i]>afn.n ,,f tlio inner gurm-luyer in the forma- 

 tion of the blood-vessels is rendered probable. 



SUMMARY. 



1. Besides the four germ-la\vrs, which have the form of 

 epithelial lamellae, special germs are developed in the higher 



ebrates 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 penetratin_ 

 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- 

 nibit a diflerence 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 

 vi>cei:il lamella? 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, 

 mid 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 folio v 

 phenomena are to be regarded : 



(a) The embryonic cells of the intermediate layer arrange 



t hemselves : 



l-'ir>t into a n. twork of cords, and 



Secondly into the substance-islands (Substan/inseln). 



(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- n<l their 



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

 {c) The Substanx.inseln become embryonic coi -ubstance 



