DEVELOPMENT 01 k 1: 4HD i5L'm>. 171 



inner germ-layer (////), numerous cells (///.v), which li>:i.-e their epi- 

 thelial character, and send out proe6886t in tin- manner <>i lymph- 



orpusdes. They soon distribute i hem-eU,', M migratory . 

 e\er\ where in the jelly. 



In the gastrula sta-e and subsequent ly. tlie cell-containing jelly 

 between tin- outer and the inner germ la veis ivproents a third sheet. 



which is distinguished from the latte.- histologically. and. according 



to the definition previously L, r i ven. cannot lie designated as a middle 

 laver ; tor l.y that detinition we understand the term to be 

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

 ment Mini hounding a surface. The jelly-like sheet is a product of 

 ?/' iii-riH-hu/i'/'v, which mny be (list inguished from them by the name 

 ."/if/ nie or intermediate /ni/n- (Zwischonblatt). 



Once termed, the meseiichyme continues to grow as an independent 

 ti>sue. in that the cells which at first migrated into the jelly at a. 

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

 y//r*r //// /////''-'/try//, continue to increase uninterruptedly by means of 

 cell -division. In its growth it pe:n-tiate* into all the interstices 

 which ari-e when the ^erm-layers, as happens in many Coelenterates, 

 produce the most complicated structures by the formation of folds and 

 rvMjLrinations; it furnishes everywhere a support for the epithelial 

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

 chvme-eells 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 beci me, as is to be seen in Ctenophores and Kehinodenus, smooth 

 muscle-crlls, 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 " Leln huch '' I set forth that the 

 whole im-si-n chyme-question in the Vertebrate-; was >till 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 RABL, of RUCKERT, ZIEGLER, and 

 V.\N WI.IHE, we have acquired more accurate explanations concerning 



