HISTORY OP THE GEEM-LAYEE THEOEY. 147 



he remarks, "it is never to be regarded as anything else than a 

 metamorphosis of tlie blastodei'm and its layers, endowed as they are 

 with an inexhaustible store of formative energy.'' A few years 

 later the germ-layer theory reached at the hands of Gael Eenst von 

 Baer a preliminary completion, which served for some time, von 

 Baer, likewise a pupil of Dollingee, had observed in Wiirzburg the 

 beginning of the investigations of his young friend, Pandee. In 

 laborious studies pursued for many }''ears, Baee followed with 

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

 morphosis into the individual organs of the adult body, principally in 

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

 and recorded his investigations in his classical work, " TJeber Entwick- 

 lungsgeschichte der Thiere, Beobaohtung und Eeflexion," which is 

 unsurpassable both in observations and in its general standpoints. 



Baer differs from Pander in maintaining that each of the 

 two primary germ-layers, which he distinguishes as animal and 

 vegetative, subsequently divides into two sheets. The animal 

 germ-layer divides itself into dermal lamella and sarcous lamella 

 (Hautschicht, Pleischschicht), the vegetative into mucous lamella 

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

 arisen. The individual organs are developed out of the germ-layers 

 by morphological and histological differentiation. 



A further advance beyond that of Baer 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- 

 struction in microscopes, methods of investigation were refined. 

 It is chiefly Remak and K6llikee who have promoted the germ- 

 layer theory in this direction. 



Eemak 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 

 tissues of the completed organs. He showed that out of the lowest 

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

 dular cells of the intestinal tiibe 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 layers 

 furnish the mechanically sustentative substances and the blood, tho 

 muscular tissue, and the urinary and sexual organs. 



In regard to the manner in which the four secondary germ-layers 

 arise, Remak differs from Baer. Out of the two primary gernn 

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



