DIFFERENTIAL DEVELOPMENTAL MODIFICATION. Ill 249 



gion, resulting in all degrees of approximation of eyes to complete cyclopia, 

 reduction in size of eyes, anophthalmia, and acephaly (Fig. 98, A-E). 

 Evagination, reduction, or absence of the stomodeum also results. Other 

 organs are also inhibited in many cases. 



Ranzi interprets cyclopia in the squid as indicating a primary median 

 position of the optic primordium, and the graded series of forms between 

 cyclopia and normal position of eyes as different degrees of inhibition of 

 its duplication and change of position, but there is no evidence in normal 

 development of a primarily median and single optic primordium. It ap- 

 pears more probable that here, as in the planarian head, susceptibility 

 decreases from the median region laterally, so that the median region is 

 most inhibited; and with increasing degree of inhibition eyes develop 

 nearer the median line or a single median eye forms. Moreover, the arms 

 of the sea-urchin pluteus show a graded series of approximations to the 

 median plane and a single median arm, and finally ventrodorsality is 

 obliterated. It is certain that the arms do not develop from a primordium 

 primarily median, but they show the same series of approximations to 

 the median plane as the eyes of the squid and the planarian. 



The hypothesis is advanced by Ranzi that differential susceptibility 

 has nothing to do with quantitative physiological or metabolic gradients 

 but that the most susceptible regions are those in which more complex 

 embryogenic processes are going on, and that these regions are more 

 susceptible only while these processes are occurring. What he means by 

 "more and less complex processes" is not at all clear. If morphological 

 complexity is meant, there is abundant evidence that his hypothesis is 

 incorrect. The apical region of an alga axis is certainly, if anything, less 

 complex morphologically than other parts, but it is more susceptible: the 

 apical region of the sea-urchin embryo does not appear to be any more 

 complex morphologically than other parts but is more susceptible. The 

 regenerating planarian head is more susceptible than parts posterior to it 

 and reduces dyes more rapidly in low oxygen from the beginning of its 

 development on, as long as the animal is in good condition. If Ranzi^ 



^ Ranzi, 1926, 1927, 192S, 1929a, b, 1931, 1932, 1938; Ranzi e Falkenheim, 1937, 1938. In 

 several of these papers it is maintained that physiological gradient pattern either does not 

 exist or is of no fundamental importance in development. Since the experimental evidence 

 presented in these papers indicates presence of gradients, the views presented are personal 

 opinions rather than conclusions from the data of experiment, and it need only be said that 

 they are not in accord with a great volume of experimental evidence. 



