formulated the existence of 'special legions in the germ, which 

 give rise to special organs.' His imagined that the ' primary 

 constituents of the organs of a chick were present in superficial 

 extension in the germinal disc,' i.e. in the cell-body of the ovum, 

 and that each organ is therefore represented by a definite part 

 of the body of the egg. As has already been mentioned in the 

 historical introduction, subsequent investigations, made in the 

 course of the following ten years, proved that the • primary con- 

 stituents ' of the various structures are to be found in the nuclear 

 substance. The special form in which His expressed his views 

 was thus certainly contradicted, although the fundamental prin- 

 ciple of his theory was not thereby affected in its general sense, 

 which indicates that the differentiating principle of ontogeny is to 

 be looked for in the cells themselves, and not in external influ- 

 ences. W ilhe lm Rou;i* was the first to prove definitely that the 

 differentiation of the.^gg into the embryo is certainly not caused 

 by influences existing aparTTromthe egg. but that it is due to 

 causes originating in the egg itself. Pfluger f showed with regard 

 to the ovum of the frog, that whatever position the egg is forced to 

 take up the upper side always gives rise to the animal pole of the 

 embryo, and it was thought that this must be due to the force of 

 gravity. Roux, however, proved that frogs' eggs which are 

 rotated slowly in a vertical direction, develop just as well as 

 thosfi_onZ\vhich the force of gravity is not interfered with. It 

 has further been proved by Born % that, although when an egg 

 undergoes development in a fixed position the substance of the 

 cell-bodv does not become displaced at first, the nucleus never- 

 theless changes its position, for it very soon passes to the upper 

 pole of the egg. at which point development then begins. 

 These observations undoubtedly proved that the formative 

 forces are situated in the egg itself; but they still left it unde- 

 cided whether the differentiation of the ovum is due essentially 

 to the action of the individual cells alone, — that is to say, 

 whether differentiation occurs independently in each individual 

 cell, so that it would, if necessary, be capable of passing through 



* Wilhelm Roux, ' Beitiage zur Entwicklungsmechanik des Embryo," 

 Miinchen, 1885. 



t Pfluger, ' Ueber den Einfluss der Schwerkraft auf die Thielung der 

 Zellen u. auf die Entwicklung des Embryo,' Arch. f. Physiol., Bd. xxxii., 

 1883, p. 68. 



X Born, ' Biologische Untersuchungen,' (I.) .Arch. f. mikr. Anat. Bd. 24. 



