296 



EMBRYOLOGY 



the cells in the living condition. However, in the case of other cells, such as 

 the amoeba, it is known that the nucleus can be removed and the cell will 

 live and move about for many days. Probably, therefore, lack of a nucleus 

 in sea urchin development is associated with lack of differentiation rather 

 than with a failure of maintenance of the living condition of the cells. This 

 question is by no means completely answered, however. 



Studies of mutation within the nucleus confirm the theory that the 

 nucleus is directly concerned with differentiation. Mutant genes control the 

 character of the egg itself, the cleavage stages, gastrulation, development of 

 limbs — in fact, every part of the body can be modified by the introduction 

 of mutant genes. 



The changes may be very specific. For example, Figure 190 shows a late 

 chick embryo which lacks wings. The gene may control specifically the 

 character of the eye or the wing or even very local phenomena, such as the 

 character of the bristles in Drosophila. The problem for the embryologist 

 is how to reconcile the local action of these mutant genes with the generally 

 assumed hypothesis that the genes in all parts of the developing embryo are 

 the same. In other words, any mutant gene affecting the eye color in 

 Drosophila is also present in the body. Why does it not change the color of 

 the body? 



The usual answer is that the genes may be the same in all parts of the 



Fig. 190. A wingless chick em- 

 bryo. The defect is caused by a 

 recessive gene which prevents the 

 formation of the wing but does not 

 affect the leg. The same gene also 

 prevents the formation of the meta- 

 nephros and the lung. (Courtesy 

 Professor Edgar Zwilling, Storrs 

 Agricultural Experiment Station, 

 University of Connecticut.) 



