112 TISSUES IN GENERAL. 



the germ of the impregnated egg of the chicken. The segmenta- 

 tion was first observed by Prevost and Dumas (1824) in the ovum 

 of frog ; by Coste (1848) in the ovum of fowls ; and by Bischoff 

 (1842) in the ovum of mammals. According to this last-named 

 observer, the subdivision into smaller elements in the rabbit's 

 germ does not go on uniformly throughout its whole extent, 

 inasmuch as in the germ a cavity is formed, around which the 

 elements of segmentation accumulate, in order to build up the 

 germ-membrane proper, with a slightly thickened spot, the 

 germ-hill of Von Baer. 



The first differentiation of the germ-disk, or the germ- 

 membrane, consists in the formation of layers, of which at first 

 two, shortly afterward three, are recognizable. The formation 

 of such layers became known first through the researches of 

 Caspar Friedrich Wolff (1768), who claimed that the whole 

 system of the intestines is developed from simple laminae. 

 Pander, in 1817, perfected the theories of Wolff ; he knew that 

 after hatching had continued for twenty-four hours, three easily 

 separable layers could be found in the germ-membrane. Von 

 Baer, in 1822, described four layers, of which the two upper he 

 termed the animal, the two lower ones the vegetative. Remak, 

 in 1855, maintained that the germ-membrane of the impregnated 

 but unhatched egg consists of two layers, and upon hatching 

 the* lower is again split into two layers, the lower of which 

 lines the one above it like an epithelial cover. Having ascer- 

 tained the individuality of each of these three layers, he 

 endeavored to find out their relation to the developing organs ; 

 he called the upper layer the horny or sensorialj the middle 

 layer the motorial and germinative j the under layer intestinal 

 and glandular. According to S. Strieker's researches (1860- 

 1870), the original under layer of Remak consists at least above 

 the germ-cavity, and before the middle layer has made its appear- 

 ance of only a single stratum of flattened cells, and the 

 formation of the middle layer is due to the immigration between 

 the two layers of new cells. He termed the upper layer of 

 Remak the combined horny and nervous layer, as he found that 

 in batrachia the horny layer is quite distinct from the nervous 

 layer, the former being uniformly thin j the latter, on the con- 

 trary, thickened even in the earliest stages in that part where 

 later the brain is formed. He is unable to confirm, despite of 

 Remakes positive assertions, that nervous elements are also 

 developed from the middle layer. 





