30 



THE STRUCTURE OF THE FERTILISED EGG 



removed before the beginning of 

 cleavage, without any disturbance 

 of their further development. 



In such cases, a normal embryo 

 originates from part of an egg. 

 However, the opposite, devel- 

 opment of a single embryo from 

 the fusion of two or more eggs, 

 has also been observed. Driesch 

 (1900) has done this experiment 

 in sea urchins, and it has been re- 

 peated and extended by Bierens 

 de Haan (1913). The envelopes of 

 two sea urchin eggs were removed, 

 and the eggs were gently pressed 

 together. They then often fused 

 into one germ of double size. Here 

 too, if certain conditions are ful- 

 filled, a single, harmoniously built 

 embryo will develop from the pro- 

 duct of such a fusion (Fig. 11). 



Mangold and Seidel (1927) have 

 done the same experiment in newts. 



Fig. 11. (a) pluteus of a 

 sea urchin, Paracentrotus 

 lividus, which has originated 

 from, the fusion of two eggs, 

 (b) the same pluteus, for 

 comparison with (c) a 

 normal pluteus drawn at the 

 same scale. After Bierens 

 de Haan. 



After removal of the envelopes, 



newt eggs at the two-cell stage often assume a dumb-bell shape. 

 When one was placed crosswise on top of the other, two of 

 these dumb-bells often fused into one single germ. Each pair 

 of opposite quadrants of such an embryo was derived from 

 one of the original eggs. Part of the products of these fusions 

 proved to be able to develop into single, more or less har- 

 moniously built embryos. This was possible even when the 

 fused eggs belonged to different species of newts so that the 

 resulting embryo was a "chimera", consisting of quadrants 

 which alternately belonged to either of the two species 

 (Plate 11). 



The view that the egg has a fixed spatial structure, and that 

 this is responsible for the development of its parts into definite 

 organs, is rendered highly improbable by these experiments. 



