A PRELIMINARY EXPERIMENTAL ANALYSIS 43 



different parts of the embryo, ^ or by changes of shape, such as 

 those which are consequent on the release of the embryo from its 

 viteUine membrane'- (see Appendix, p. 481). 



With regard to the actual paths of displacement followed by the 

 invaginated tissues during these "mass movements" which bring 

 about gastrulation, it may be said that the nearer any given piece of 

 tissue is to the dorsal lip of the blastopore at the outset of gastrula- 

 tion, the farther forward in the embryo will it find itself when that 

 process is completed. So, those cells which occupy the place where 

 the invagination first forms become the front wall of the fore-gut ; 

 those cells of the marginal zone in the mid-dorsal line which are the 

 first to be tucked in form the tip of the notochord (figs. 3 and 4). 



§5 

 Attention may now be turned to the presumptive regions of the 

 future organs. As has been shown in Chap. 11, these regions can be 

 mapped out on the blastula, although there are no visible limits to 

 distinguish them. The question arises as to how these various 

 regions have their respective fates allotted to them. 



The first point to make clear in any discussion of the origin of 

 differentiation is the fact that it is impossible to appeal to differ- 

 ences between the nuclei of the cells of the blastula in order to 

 account for the eventual differentiation of those cells. By making 

 eggs undergo cleavage under compression between glass plates, 

 the normal regular sequence of directions of cleavage can be dis- 

 turbed, so that the nuclei come to be situated in cells other than 

 those in which they would find themselves in normal unhindered 

 cleavage. Nevertheless, the development of embryos so treated and 

 then released from pressure is normal, and it is therefore clear that 

 it is quite immaterial whether any given nucleus finds itself in one 

 particular cell or in another.^ This is confirmed in other ways 

 and on other forms (see p. 85 and fig. 36). 



This means that there is no inequality in nuclear division during 

 early cleavage, and it is therefore impossible to attribute any deter- 



1 Huxley, 1927; Vogt, 1928 b; Gilchrist, 1928, 1929; Dean, Shaw, and 

 Tazelaar, 1928; Tazelaar, Huxley, and de Beer, 1930; Castelnuovo, 1932. 

 ^ Spemann, 193 1. 

 ^ Hertwig, 1893; Spemann, 1914, 1928. 



