248 



PATTERNS AND PROBLEMS OF DEVELOPMENT 



tional development in planarians and annelids suggests existence of some- 

 what similar physiological patterns in the longitudinal axis. The problem 

 of bipolar forms will be considered in a later chapter. 



In several gasteropod species with free-swimming veliger larvae de- 

 velopment of velum and shell gland have been inhibited by external chem- 

 ical agents. In the inhibited forms the apical region remains smaller than 

 in normal larvae, and the prototroch persists as a simple circular girdle 

 of ciliated epithelium ; even though the larvae remain alive for much longer 

 periods than necessary for development of shell gland and spiral coiling 



Fig. g8, A-E. — Differential inhibition in a cephalopod (after Ranzi, 1928, 1929(7) 



and velum, these may all be completely inhibited, the larva remaining in 

 the primary trochophore stage (Child, unpublished). 



Differential inhibition of development in the cephalopod Loligo vulgaris 

 and other species has been described by Ranzi.' The modifications were 

 produced chiefly by LiCl, although the author states that MgCU and 

 ultra-violet radiation produced similar effects. As with other forms, the 

 individual modifications differ according to stage of development at 

 which embryos are subjected to the agent, length of exposure period, con- 

 centration, and susceptibility of the individual. The most conspicuous 

 feature of the inhibited forms is the differential inhibition of the head re- 



' Ranzi, 1926; 1927; 1928; 19290, b; 1931; 1932. 



