

\V I! AMI I!I.M.1. 1 < .) 



of migratory <ell>. is perhaps IM-SI shown in tin- investigation of 

 ii-ai: mlu-yes of Jinny Kishe>. "One sees di>tii:ctly." llm> 



\\'r\Ki;i:.M M describee it. "how tlic cells by means of aimeboid 

 ni) r ions. ai.d of somet i ra<>rdinarily long protopla>inie ].r>- 



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

 bryo and upon 1 In- yolk, which is not yet cloth-.! with liypnhlast, 

 and creej) toward deiinite places, as it' they actel voluntarily ami 

 iously.'' By virtue .f this peculiarity, the mesenchyme-cells 

 vdy penetrate into all larger and smaller figure- which 

 een 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 

 a.v nerves. 



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

 Driven an essentially different presentation of the development of the ii!e.-en- 

 Formerly, supported by the investigations of His. WAI. I>I;YKI;. KOLL- 

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

 rliit-f source of the mesenchyme to a limited territory of the germ, to the an-a 

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

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

 various regions of the middle germ-layer. Thus 1 come back again to an in- 



tation whi'-h 1 had already propoundedas probable in ki Die Coelomtheorie " 



Hand "Die Kntwiokiflun-r d-s mittleren Keimblattes " (p. 122), to the 

 inti-rpn-tation, n;uncly, that nicscnchyine-.<:enns 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 :.tial part of that theory con>ists in this, 



; establishes in the earliest development of ti.-sue a c -..ntrast between 

 the epithelial LTcrni-layers and a packing li>sue, produced by a dissolution of 

 the epithelial continuity, which spreads itself out between the germ-la; 



<m appears a- an independent structure. 



Indeed, \\ith this theory u a i>a>K it would not be surprising \ltJu-jtro- 

 duct ion of netenekjfOtatie fi.^ni' xhonld not l>c limited simply to the middle fjerm- 

 l<:i/>'r, and if tin- entodcrm //// tlic contribution of cell-material sltould particijtate 

 in its formation. 



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



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

 heading i> one of the most obscure in the realm of comparative 

 embryology. The very invr.-tigators who have endeavored ni"-t 

 rei ently and with the mo-t 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 



