REMAK 209 



in the second volume of his Entwickelungsgeschichte (1837), 

 where he says with reference to the separation of the blasto- 

 derm of the chick into two layers. " Yet originally there are 

 not two distinct or even separable layers, it is rather the 

 two surfaces of the germ which show this differentiation, 

 just as polyps show the same contrast of an external surface 

 and an internal digestive surface. In between the two layers 

 there is in our germ as in the polyp an indifferent mass " 

 (p. 67). The terms ectoderm and entoderm were introduced 

 by Allman 1 in 1853 for the two cell-layers in the Hydrozoa. 



Remak is the second great name in the history of the 

 germ-layer theory. He had the great advantage over von 

 Baer of being able to make use of the cell-theory in inter- 

 preting the formation of the germ-layers. Microscopical 

 technique also had been greatly improved since i828. 2 



Remak's greatest service was that he put the germ-layer 

 theory in direct relation with the cell-theory by demonstrating 

 the cellular continuity from egg-cell to tissue, and by showing 

 that each germ-layer possessed distinctive histological 

 characteristics. Hardly less important was his clear 

 marking-off of the "middle layer" as a separate and distinct 

 layer of the germ. He it was who introduced the modern 

 conception of the mesoderm, and cleared up the confusion in 

 which Pander and von Baer had left the organs formed 

 between the serous and the mucous layer. Remak's middle 

 layer was a different thing from Pander's ill-defined "vessel- 

 layer"; it included and unified from a new point of view the 

 " vessel " and " muscle " layers of von Baer. 



There are in the unincubated blastoderm of the chick, 

 according to Remak, 3 two cell-layers, of which the undermost 



1 Phil, Trans., cxliii., p. 368, 1853. 



' The principle of achromatism was discovered (by Fraunhofer) and 

 achromatic microscopes introduced in the early part of the igth century. 

 The use of chemical reagents, such as acetic acid, and various hardening 

 fluids, came into fashion not long after. J. Muller seems to have been 

 one of the first to realise their importance. Remak himself invented 

 one or two fixing and hardening mixtures (pp. 87, 127, 1855), which 

 enabled him to cut excellent hand sections. Section-cutting machines 

 were not. invented till later (V. Hensen, 1866, His, 1870;. 



3 Untersuchungen iiber die Entwickclung der Wirbelthiere, folio, pp. 

 xxxvii + 195, 12 plates, Berlin, 1850-1855. 



