BISXOBT OF THE <.I.!!M I.\Vi:iC THEORY. 147 



he remarks. " it is never to be regarded as anything an a 



int'tamoi-plio-is of tin' /lA.-.v/. .././/// <(n>/ it* layers, endowed as they are 

 with an inexhaustible store of format iv energy." A few years 

 later the germ-layer theory reached at the hands of CAUL I 

 BAKU a preliminary completion, which .-< DBA. fOI 



UU.K. like\\i-e a pupil of DI>U.IM;KK, had obsTvd in Wiir/.hurg the 

 beginning of the investigations of his young friend, i In 



laborious studies pursued for many years, BACH followed with 

 wonderful accuracy the origin of the germ-layers and their i 

 morpho.xis into the individual organs of the adult body, principally in 

 tin- case of the Chick, but also in the case of some other Vertebrates, 

 ami recorded his investigations in his classical work, " Ueber Kntwick- 

 lungsgeechichte der Thiere, Beobachtung nnd Reflexion," which is 

 unsurpassjible both in observations and in its :.MiiTal standpoiir 



BAKU diilcrs from PANDER in maintaining that each of the 



t\\o primary germ-layers, which lir di>tinguishcs as animal and 



aiivo, subsequently divides into two sheets. The aniim.l 



layer divides itself into dermal lamella and sarcous lamella 



< Hautschicht, Fleischschicht), the vegetative into mucous lamelki 



and vascular lamella, so that now four secondary : -mi-layers have 



at i>-n. The individual organs are developed out of the germ-layers 



by morphological and histological differentiation. 



A further advance beyond that of BAKU could not be attained 

 until, with the establishment of the cell-theory, entirely new points 

 of view were introduced into morphology and, with improved con- 

 st ruction in microscopes, methods of investigation were refined. 

 It is chit'ily UKMAK and KOLLIKER who have promoted the germ- 

 layer theory in this direction. 



REMAK took in hand successfully in his noted investigations on 

 the development of Vertebrates the very important question, how 

 the originally similar cells of the germ-layers are related to the 

 tisanes of the completed organs. He show.-d that out of the lov 

 of the four germ-layers there proceed only the epithelial and glan- 

 dular cells of the intestinal tube and its appendages, that from the 

 uppermost layer the epithelial cells of the epidermis, the sensory 

 organs, and the nervous tissue arise, whereas the two middle la; 

 furnish the mechanically sustentative substances and the blood, tho 

 musculai tissue, and the urinary and sexual organs 



In regard to the manner in which the four secoii m-layers 



ari>e, REMAK differs from BAER. Out of the two primary germ- 

 layers he first makes a third one, the middle germ-layer, arise, and 



