SECT. 3] AND ORGANISATION 567 



Such correlations, in the position in which we now are, are difficult. 

 It is also very difficult to bring together the work of the "Entwick- 

 lungsmechanik " school and the results of physico-chemical work, 

 but close attention must be given to it in order to orientate 

 the point of view adopted in this book. Any exhaustive discussion 

 of it is rendered unnecessary in view of the papers by Huxley 

 and Spemann, and the excellent book of de Beer. Other recent 

 reviews of experimental embryology are those of de Beer; Przibram; 

 Mangold; Brachet; Weiss; Gilchrist; Hogben and Schleip. 



It will not be possible here to do more than indicate a few of the 

 outstanding problems to which modern experimental embryology 

 has supplied tentative but fairly certain answers. 



3-5. Pluripotence and Totipotence 



One may first ask, therefore, what answer has been given to the 

 question of organisation in the unfertilised egg-cell, or, in other 

 words, what modicum of preformationism are we still obliged to 

 admit into our conception of the undeveloped egg. In the very 

 earliest stage of development two important critical points occur, 

 or two stimuli are received, the first causing the egg to become 

 radially symmetrical about an axis, and the second causing it to 

 become bilaterally symmetrical in any one of an infinite number 

 of planes passing through that axis. What these processes actually 

 are is entirely unknown, although we can make a guess, as will 

 later be seen. But the fact that they have to occur before differentia- 

 tion and growth can begin makes it impossible to speak of the spatial 

 arrangement of already differentiated material in the egg, which was 

 the basis of the classical preformationism. It is now correct to say 

 that heredity does not account for the individual but for a number 

 of potentialities, some of which are brought into being in the indi- 

 vidual. The only predetermination which exists is the assurance that, 

 if the potentialities in question are brought into actuality, they will 

 produce an organism belonging to the same species as its parents. 

 The appHcation of chemical methods to the undeveloped egg will 

 probably thus in the future not be so histochemical as would have 

 been the case if an arrangement of material had existed there. 

 Nevertheless, what has been said apphes to the vertebrate egg only; 

 there are invertebrate eggs, such as those of Cerebratulus , which do 

 seem to have an intracellular determinate arrangement. The state- 



