INTRODUCTION 



symmetry are probably localized partly in the more solid outer 

 layer, or cortex of the egg, often in the form of gradient-fields 

 (Ch. IV). Next, certain components of the egg cytoplasm (so- 

 called determining substances) begin to accumulate locally 

 under the influence of these cortical factors. This leads to the 

 onset of heterogeneity in the egg, for now its parts begin to 

 vary in chemical composition. This process is called the chemo- 

 differentiation of the egg (Ch. V). The variation in chemical 

 composition within the egg influences other properties of the 

 protoplasm as well, such as permeability, metabolism, etc. 

 Moreover, the determining substances begin to interact with 

 one another. In this way the spatial multiplicity of the egg- 

 system increases rapidly. Now the genes, the carriers of the 

 hereditary properties which are localized in the nucleus, begin 

 to intervene. In the course of cleavage, the zygote nucleus 

 which arose from the fusion of the nuclei of egg and sperm 

 has divided into a great number of segmentation nuclei. These 

 come to be situated in parts of the cytoplasm which vary in 

 physicochemical composition as a consequence of chemodiffer- 

 entiation. Consequently in each segmentation nucleus certain 

 genes are "activated", whereas others remain inactive. The 

 activated genes begin to interact with the cytoplasm surround- 

 ing them. Part of the substances produced by this interaction 

 remains in the cells, and determines their further development. 

 Another part, however, diffuses from the cells, and exerts its 

 influence in other parts of the body. These latter substances 

 have been called gene hormones (Ch. VI). Under the influence 

 of the ever-increasing chemodifferentiation, divergent speciali- 

 zation takes place in the motility of the protoplasm of the 

 various cell groups. The different parts of the blastula vary in 

 this respect, and consequently a system of shifts of cell groups 

 occurs, which we have already named the topogenesis of the 

 embryo (Ch. VII). This in the first place moves the material 

 for the future organ primordia to the appropriate places in the 

 embryo. Secondly, however, it establishes direct contact between 

 cell groups of different physico-chemical composition which 

 were spatially separated at first. These cell groups thereby 

 influence each other (induction), and this interaction results 



