DEVELOPMENT OF CONNECTIVE SUBSTANCE AND BLOOD. 175 



of migratory cells, is perhaps best shown in the investigation of 

 transparent embryos of Bony Fishes. " One sees distinctly," thus 

 WENKEBACH describes it, ^how the cells by means of amoeboid 

 motions, and of sometimes extraordinarily long protoplasmic pro- 

 cesses, move themselves about independently in the body of the em- 

 bryo and upon the yolk, which is not yet clothed with hypoblast, 

 and creep toward definite places, as if they acted voluntarily and 

 consciously." By virtue of this peculiarity, the mesenchyme-cells 

 actively penetrate into all larger and smaller fissures which exist 

 between the germ-layers and the fundaments of organs which have 

 arisen from them. Everywhere they form a filling and connecting 

 mass between these structures, which afterwards acquires a still 

 greater importance as the bearer of blood- and lymph-courses as well 

 as nerves. 



In comparison with the earlier editions of the " Lebrbuch," I have here 

 given an essentially different presentation of the development of the mesen- 

 chyme. Formerly, supported by the investigations of His, WALDEYEK, KOLL- 

 MANN, and others on meroblastic eggs, I thought it necessary to refer the 

 chief source of the mesenchyme to a limited territory of the germ, to the area 

 opaca, and made the cell-material arise by delamination from the entodermic 

 layer, especially from the yolk-wall. But now I assume a manifold origin from 

 various regions of the middle germ-layer. Thus I come back again to an in- 

 terpretation which I had already propounded as probable in " Die Ccelomtheorie " 

 (p. 80) and "Die Entwickelung des mittleren Keimblattes " (p. 122), to the 

 interpretation, namely, that mesenchyme-germs in Vertebrates are perhaps 

 formed by an emigration of cells at several distinct places at the same time. 

 Whether this or that be the real mode, the essence of the mesenchyma-theory 

 is not thereby affected, for the essential part of that theory consists in this,, 

 that it establishes in the earliest development of tissue a contrast between 

 the epithelial germ-layers and a packing tissue, produced by a dissolution of 

 the epithelial continuity, which spreads itself out between the germ-layers,, 

 and soon appears as an independent structure. 



Indeed, with this theory as a basis, it would not be surprising if the pro- 

 duction of mesenchy matic tissue should not be limited simply to the middle germ- 

 layer, and if the entoderm by the contribution of cell-material should participate 

 in its formation. 



B. The Origin of the Vascular Endothelia and the Blood. 



The question of the origin of the tissues represented in the above 

 heading is one of the most obscure in the realm of comparative 

 embryology. The very investigators who have endeavored most 

 recently and with the most reliable methods to elucidate this matter 

 do not hesitate to emphasise the uncertainty in the interpretation 

 of the conditions presented to them. Even the lowest Vertebrate^ 

 which is distinguished by the greater simplicity of its structure, and 



