HISTORICAL ACCOUNT OF EMBRYOLOGY. 525 



peripheric parts of the germinal area. Rernak endeavoured at the 

 same time with the anatomical independence of the germ layers to 

 establish the relation to the various organs during the further develop- 

 ment of the organism. The names that he applied to the lamina are 

 self-explanatory. The upper lamina, as has been already stated, he 

 termed the corneal or sensorial layer (Hornblatt oder Sinnes blatt) ; 

 the middle, the motorio - germinal (motorisch - germrnatives) ; and the 

 inferior lamina, lastly, the intestinal-glandular layer (Daruidriisen- 

 blatt), since the epithelium of the intestinal system, with the associated 

 glandular organs, is developed from it. 



His,* again, admitted the existence of only a single layer of corpuscles 

 in the fertilized but non-incubated ovum, which constituted the rudiment 

 of the upper germ layer (Archiblast or Neuro-blast). The inferior layer, 

 he considered, develops during the first hours of incubation of the egg 

 by the elongation and junction of the (subgerminal) processes which 

 project from the inferior surface of the upper layer, and consist of one 

 or several rows of cells. The lower layer, on this view, therefore is 

 originally a production of the upper. From these two laminae the 

 whole embryo, according to him, is formed, with the exception of the 

 blood-vascular system and the group of the connective tissues which 

 develop from the so-called white yolk. In the middle region of the 

 germinal area, moreover, a layer separates from the upper, and one 

 also from the lower lamina ; and lastly, an axial connecting band forms 

 between the upper and lower germ layers. 



According to Hensen,f the division of the blastoderm into layers 

 occurs at a later period than was admitted by Remak. Hensen also 

 described, as forming at that period of the development of the embryo 

 when the chorda dorsalis has developed in the centre of the germinal 

 area, a "peculiar, consistent, non-nucleated membrane," to which he 

 applied the name " membrana prima." It lies between the epi- and 

 meso-blast, and is more closely adherent to the former than to the 

 latter. He considers it to play an important part in the development 

 of the embryo. 



According to Dursy,J the centre of the blastoderm (the embryonic 

 shield) consists, at about the fifteenth hour of incubation, of two layers, 

 and the inferior, he believes, may be regarded as the rudiment of the 

 middle germ layer. The statement made by Remak, that the mesoblast 

 is produced by a dilamination of this inferior layer of the blastoderm, 



* Loc. cit. 



t Yirchow's Archiv, Band xxx. ; Max Sclmltze's Archiv, Band iii. 



J Der Primitifstreif des Huhnchens. 



