396 DISCOVERY REPORTS 



It will be noted that Weill's micron-estimations are higher than those given here both for stenoteles 

 and isorhizas. This may reflect true variation between individuals or populations of Physalia. Semal 

 (1954a) found such variation between different populations of Hydra. Even in an individual Physalia 

 the typical capsule diameter of the large isorhizas may differ by as much as 5^ between two 

 tentacles. 



Developing stages can easily be obtained by peptic digestion from the ampullae connected with the 

 bases of the tentacles. The young nematocysts nearly always exhibit what Weill has shown to be the 

 prematurely discharged condition. Earlier interpreters of nematocyst development have been 

 hampered by failing to appreciate this fact. In the present study, using only fixed material, the author 

 has not attempted to reinterpret the development picture. The most recent account referring to the 

 process in Physalia is that of Will (1929). 



Developing nematocysts are found in the float, gastrozooids and palpons as well as in the ampullae. 

 In silver preparations the capsules and discharged tube (Will's ' zufuhrender Kanal') stain heavily in 

 silver preparation (PI. XXVIII, fig. 4). The group shown is typical in that it consists of a small, even 

 number of developing stages. Groups of two, four and eight are so common that one can probably 

 assume that such groups derive from one or two primordial cnidoblasts which divide in situ by 

 mitosis before capsule secretion begins. The close grouping of the cnidoblasts can often be attributed 

 to the presence of bridges of cytoplasm holding the cells together. Such bridges have been described 

 in the older literature (for example, in Physalia by Goto, 1895) and more recently in Hydra (Hess, 

 Cohen and Robson, 1957). In silver preparations one can often make out, especially in the younger 

 stages, a strand of fibrous material running between the two nuclei within the cytoplasmic bridge 

 (PI. XXVIII, fig. 3). This fibre strand is in all probability a mitotic spindle relic — the 'fusom' of 

 J. Hirschler (1935). In certain 'fusoms' chromatin material lodging in the interchromosomal con- 

 nectives during anaphase gives to the persistent ' fusom ' a nuclear reaction. Such cases have been 

 interpreted as stages in amitotic division. Some earlier students of cnidoblast development appear to 

 have been misled by this ' pseudoamitosis '. 



In the more mature cnidoblast groups the ' fusom ' appears to degenerate but the cytoplasmic bridges 

 persist longer, and are sometimes seen in the fully mature groups. Hirschler (1955) describes a case 

 (egg nurse-cell complexes in the Arthropod ovary) where the 'fusom' provides a means whereby 

 physiological co-operation between members of the group can take place. It would be interesting to 

 ascertain whether any such co-operation were possible within the cnidoblast groups in Physalia. In 

 cases where the group remains interconnected in maturity one might look, for instance, for some 

 capacity for synchronized development or simultaneous discharge. A 'fusom '-like structure also 

 occurs in interstitial cells of Hydra (McConnell, 1937). In Physalia, the 'fusom' is not confined to 

 the cnidoblast groups. It occasionally occurs in the float musculature but is only easily detectable 

 where it contains Feulgen-positive material. This is probably matrical matter from the chromosomes 

 which has become lodged in the interzonal connectives during anaphase. Examples of such anaphases 

 also occur in the float. 



Some simple experiments were carried out on living material in the Canary Islands to determine 

 some of the factors influencing nematocyst discharge. 



Parker and Van Alstyne (1932) mention that the tentacle nematocysts in Physalia discharge in an 

 electrical field, and that discharge is inhibited by magnesium anaesthesia. The author independently 

 arrived at the same results. An attempt was made, by controlling the ' electrical field ', to compare 

 discharge thresholds in different regions and under various conditions. The electrodes were placed at a 

 fixed distance from the tentacles, and the strength of the shock was increased until the nematocysts 

 discharged en masse. A 2 V. accumulator was used, giving make-and-break shocks through an induction 



