AS THE ELEMENTAL GERM OF AX ORGANISM 855 



respond to an organ which develops later, or to part of snoh an 

 organ, and on the other hand, every organ developed from the 

 germinal area must have its preformed germ in a definite region 

 of this area. The material for the germ is already present in the 

 flat germinal disc, but it is not morphologically distinct, and hence 

 is not to be recognised as such at this stage. By tracing the 

 mature organs back to their elemental form, we shall be able to 

 discover the situation of each during the period of incomplete 

 morphological separation, and indeed, if we wish to be consistent, 

 we must apply this method to the fertilised and even to the un- 

 fertilised ovum also." 



It is hardly necessary to emphasise how sharply opposed this 

 principle of the formation of organs in the germinal area is to the 

 above-mentioned theory of heredity. One of the first points to 

 be noticed is, that the influence of the paternal elemental germs, 

 upon the formation of the embryo, is entirely left out of account. 

 For this reason alone, the theory is evidently untenable. But, in 

 addition, various experimental facts, which, as Pfliiger has pointed 

 out, indicate that the egg is isotropous, entirely disprove it. 



By the term isotropy of the egg, Pfliiger (VII. 50), wishes to 

 imply, that the contents of the egg are not arranged in such a 

 manner as that the individual organs can be traced back to this 

 or that portion of it. He draws his conclusions from experiments 

 made upon Frog's eggs. The Frog's egg is composed of two 

 hemispherical portions, one of which, the animal half, is pig- 

 mented black, whilst the other, or vegetative portion, is clear or 

 colourless, and is, at the same time, specifically heavier. In conse- 

 quence of this difference in specific gravity, the eggs, immediately 

 after fertilisation, assume a definite position in the water, the 

 pigmented portion always being directed upwards, so that the 

 egg-axis, which connects the animal with the vegetative pole, is 

 vertical. It is possible, however, to experimentally force the 

 eggs which have just been fertilised to take up an abnormal 

 position, that is to say, to prevent them from rotating in the yolk- 

 membrane by applying friction to it. The experimenter, for 

 instance, can force the egg to assume such a position that the 

 egg-axis shall lie horizontally, instead of vertically. Now when 

 the process of division begins, the first division plane, in spite of 

 the changed position of the egg, is in a vertical direction, for its 

 position depends on that of the nuclear spindle, as shown on p. 219. 

 As Born (IX. 37), has minutely described, however, although the 



