136 THE PHYSIOLOGY OF EARTHWORMS 



formed until once again the hormonal balance is restored and 

 growth ceases. 



These facts do not, however, explain w^hy the nervous system 

 must be intact to induce regeneration within a particular segment, 

 unless secretory products tend to be released locally from circula- 

 tion throughout the body. In the latter case stimulatory substances 

 could be distributed to segments at a distance from the site of 

 origin and thus be stimulated. 



This explanation should be compared with the hypothesis of the 

 development of a critical inhibitory voltage due to Moment (1949) 

 and Kurtz and Schrank (1955) (see Chapter VIII). 



A sub-terminal centre in the posterior region of the body has 

 been suggested by Needham (1958) on the basis of studies in the 

 nitrogen excretory pattern during regeneration. But apart from the 

 segmentally repeated neurosecretory cells of the ventral nerve 

 cord (Aros and Vigh, 1959) there is no histological or cytological 

 evidence at the moment for this. 



There is no information at the moment as to the chemical nature 

 of the products of these secreting cells. Pharmacological studies 

 point to the implication of adrenaline, acetylcholine, and possibly 

 5-hydroxytryptamine as transmitter or more widely acting 

 chemicals deriving from the nervous system (see Chapter XI), but 

 nothing has so far implicated these substances in the sexual or 

 regenerative growth processes. Schneidermann and Gilbert (1958) 

 found that extracts from L. terrestris contain substances that have 

 insect juvenile hormone activity, maintaining pupal skeleton in a 

 treated area when Anthereae polyphemus metamorphoses to the 

 adult moth. But the structure of this active substance is unknown, 

 and there is no evidence as to what, if anything, it does in the 

 earthworm. 



Summary 



Neurosecretory cells were early discovered in Liimhricus and 

 their presence confirmed many times since. Various types of 

 neurosecretory cells have been described and some of these have 

 been associated with physiological functions. In particular, effects 

 of the secretory areas have been noted on activity of the repro- 

 ductive organs, egg laying and cocoon production, with possible 



