DEVELOPMENT OF CONNECTIVE SUBSTANCE AND BLOOD. 171 



inner germ-layer (hy), numerous cells (ms), which loose their epi- 

 thelial character^ and send out processes in the manner of lymph- 

 corpuscles. They soon distribute themselves as migratory cells 

 everywhere in the jelly. 



In the gastmla -stage and subsequently, the cell-containing jelly 

 between the outer and the inner germ-layers represents a third sheet, 

 which is distinguished from the latter histologically, and, according 

 to the definition previously given, cannot be designated as a middle 

 germ-layer ; for by that definition we understand the term to be 

 limited to a sheet of embryonic cells, having an epithelial arrange- 

 ment and bounding a surface. The jelly-like sheet is a product of 

 the germ-layers, which may be distinguished from them by the name 

 mesenchyme or intermediate layer (Zwischenblatt). 



Once formed, the mesenchyme continues to grow as an independent 

 tissue, in that the cells which at first migrated into the jelly at a 

 definite stage of development, to which one may give the name 

 mesenchyme-germ, continue to increase uninterruptedly by means of 

 cell-division. In its growth it penetrates into all the interstices 

 which arise when the germ-layers, as happens in many Coelenterates, 

 produce the most complicated structures by the formation of folds and 

 evaginations ; it furnishes everywhere a support for the epithelial 

 layers which repose upon it. At the same time some of the mesen- 

 chyme-cells can alter their original histological character as simple 

 trophic or nutritive cells of the intermediate substance. Thus here 

 and there they differentiate contractile substance at their surface, 

 and become, as is to be seen in Ctenophores arid Echinoderms, smooth 

 muscle-cells, the ends terminating either in one fine point, or 

 dividing themselves into several processes, as is more frequently the 

 case with Invertebrates. 



In Vertebrates also, after the two primary germ-layers have arisen, 

 a process similar to that which we have just considered appears to 

 lead to the formation of connective tissue and blood, two tissues 

 which correspond morphologically and physiologically to the mesen- 

 chyme of Invertebrates. 



In the first two editions of the " Lehrbuch " I set forth that the 

 whole mesenchyme-question in the Vertebrates was still in a nascent 

 condition, that the account therefore presented nothing final, but 

 bore in many respects the character of the provisional. Since that 

 time an essential advance has been made in this field. Thanks to 

 the investigations of HATSCHEK and EABL, of RUCKERT, ZIEGLER, and 

 VAN WIJHE, we have acquired more accurate explanations concerning 



