92 THE TOPOGENESIS OF THE EMBRYO 



egg consists of a number of cleavage cells, or blastomeres, each 

 containing one cleavage nucleus. A close correlation nearly 

 always exists between the direction of the first cleavages, and 

 the polarity and symmetry of the egg, as determined by its co-' 

 ordinate system. As a rule, the first two cleavages are in a 

 meridional plane, and at right angles to each other, whereas 

 the third cleavage lies in the equatorial plane ^). From this 

 point onwards, cleavage may follow several different courses. 

 Sometimes it remains very regular so that the cleavage planes 

 always cut through the egg in a definite direction, and two eggs 

 of the same species in the same stage have identical structures. 

 Evidently, the direction of cleavage is rigidly determined in 

 such cases. The eggs of most molluscs and annelid worms may 

 serve as examples (see above, p. 51). In other cases, cleavage 

 soon becomes more or less irregular. The direction of the 

 cleavage planes does not follow any general rule, and is ap- 

 parently more or less independent of the polarity and symmetry 

 of the egg. These differences are not essential for the further 

 course of affairs, and they can be neglected here. 



During cleavage the egg cytoplasm is distributed between 

 the blastomeres. As a consequence of the previous chemo- 

 differentiation, the cytoplasm of the various blastomeres will 

 be physically and chemically different. We have already seen 

 how this influences the realisation of the nuclear factors by 

 the activation of different genes in different cells. We have also 

 discussed (p. 62) how differences in the developmental potencies 

 of the cells arise thereby. In other words, the germ now becomes 

 a complex of cell groups with divergent potencies. The necessary 

 conditions are thereby provided for the processes which now 

 follow, in the course of which the embryo is formed (embryo- 

 genesis). This consists of two phases: the development of form, 

 or topogenesis, and the differentiation of the tissues, or 

 histogenesis. 



Topogenesis is a series of movements, resulting in cell 

 migrations, which bring the prospective tissues of the body 



1) Ascaris is an exception; here, the first cleavage is equatorial (Fig. 25). 

 See also p. 70 on cleavage in insects. 



