202 COJMPARATIVE PHYSIOLOGY 



into a complete larva ; but if the constriction is incomplete 

 double-headed forms result. Double-headed monsters can 

 be produced from frog's eggs by inverting them in the two 

 cell stage, or (Bellamy) by the action of cyanides in appropriate 

 concentration at a later stage in development. Lastly may 

 be mentioned the production from sea urchins by Herbst 

 (1893) of plutei vi^ithout arms or spicules by exposing the egg 

 to the action of potassium salts. 



Pioneer work of this kind provided a wealth of spectacular 

 instances of ways in which differentiation and individuation 

 can be partially controlled in a predictable manner by experi- 

 mental procedure. But in many cases the characteristic 

 abnormalities were on subsequent examination found to be 

 procurable by such a variety of methods as to defy analysis. 

 Thus Stockard's cyclopean embryos, thought at first to be 

 due to the specific action of the magnesium ion, can be obtained 

 with alcohol and other very different reagents. McLendon 

 (19 1 2) produced cyclopia in fish embryos with isotonic 

 solutions of lithium chloride, sodium hydrate, and a number 

 of other equally dissimilar substances. 



During the past few years a new impulse has been given 

 to experiment on these lines by a hypothesis which has been 

 elaborated by Child. The evidence brought forward by Child 

 and his co-workers in favour of his hypothesis can hardly 

 as yet be said to be crucial. But its effect has been to introduce 

 new concepts v/hich, whether the main body of this work 

 stands or falls, are bound to simplify the nature of the problems 

 of individuation and prove the starting-point of new lines of 

 investigation. Of these concepts, it is not least im.portant 

 that Child, by emphasising the idea of polarity and describing 

 the architecture of the organism with reference to axial sym- 

 metries, has provided us with the very useful term, axiate 

 pattern. The arrangement of structural parts in the higher 

 organisms is so immensely complex that one must limit the 

 field in order to make the search thorough. One way of doing 

 this is to confine attention to the arrangement of parts with 

 reference to som^e axis of symmetry, e.g. the oral-aboral axis 

 of the body. 



