20 THE INITIATION OF DEVELOPMENT: 



cytoplasm (Fig. 6). Each chromosome divides into two halves, 

 but the halves remain together, and finally all the chromosomes 

 reunite into one nucleus in the centre of the now very much 

 enlarged aster. This process may repeat itself several times, 

 but irregularities soon appear, and, finally, this so-called mon- 

 astral cycle terminates in the death of the eg^. 



In frog eggs, which have been pricked, but not inoculated with 

 cell material, we again find a cortical reaction with elevation 

 of a fertilisation membrane, followed by a monastral cycle, 

 which in the end comes to a stop, culminating in the death of 

 the egg. 



Evidently, certain stimuli release a chain of reactions in the 

 egg, called the activation of the egg. The most conspicuous 

 links of this chain are the elevation of the fertilisation mem- 

 brane, and the monastral cycle. These processes, however, do 

 not result in normal development, but lead into a blind alley, 

 ending with the death of the egg. For normal development to 

 occur, other factors must bring the egg cytoplasm into such a 

 condition that, instead of the monastral radiation, a dicentric 

 radiation, or amphiaster, can be formed. The latter leads to 

 normal division, and thereby to cleavage of the egg. Bataillon 

 has named this process ''regulation''. In Loeb's method, the 

 treatment with hypertonic sea water is the regulating factor; 

 in Bataillon's method the inoculation of cell material acts as 

 such. 



Consideration of these two groups of processes shows that 

 widely divergent means can be used for the activation of the 

 egg in different species. Both mechanical stimulation (pricking) 

 and physical treatment (illumination, induction shock, increase 

 or decrease of temperature, or of osmotic pressure) will serve 

 the purpose. Chemical treatment especially, however, has been 

 applied on a large scale (various acids, salts, and alkalis, but also 

 non-electrolytes, such as urea, saponin, etc.). This variety of 

 methods tends to show that none of them constitutes the 

 specific, natural agent itself. The inherent properties of the egg 

 itself must be responsible for the fact that it reacts in the same 

 way to such entirely different stimuli. Now the results of 

 several investigations indicate that calcium ions play a special 



