DISCUSSION 



85 



swollen endings of neurosecretory cells that 

 synthesize the hormone. 



Krishnan (1954) has studied the effects of eye- 

 stalk removal on the rate of tissue respiration. 

 Three days after performing this operation on 

 Carcinus maenas, he found the respiratory rate 

 of muscle from animals without eyestalks to be 

 decidedly lower than that of muscle from unoper- 

 ated crabs. Krishnan noted, nevertheless, that 

 when all his data were plotted, the resulting 

 curves for the two groups of animals were simi- 

 lar. They differed mainly in that the curve for 

 unoperated crabs was displaced to the right of 

 the curve for operated crabs by a distance 

 representing six days. In other words, compara- 

 ble decreases (and subsequent increases) in 

 respiratory rate of muscle occur in operated 

 and unoperated animals, although in the latter 

 only after a lag of six days. Krishnan did not 

 offer an explanation of these observations. The 

 levels may be related to the stage of the animals 

 in their molt cycle. If the unoperated crabs were 

 gradually approaching molt, the respiratory rate 

 of their muscle would reflect this and would 

 yield a curve resembling that of premolt crabs 

 without eyestalks. Because eyestalk removal 

 accelerates molt-preparatory processes, how- 

 ever, the two curves would remain separated in 

 time. 



It may be noted that Bliss (1953), working 

 with whole specimens of Gecarcinus lateralis, 

 found the respiratory rate to be high immediately 

 after eyestalk removal and to remain high 

 throughout the entire period preceding ecdysis. 

 Thus a difference exists between the respira- 

 tory rates of animals without eyestalks and of 

 their isolated tissues during the premolt period. 



As mentioned above, a molt-inhibiting hor- 

 more occurs in the eyestalks of crustaceans. In 

 the preparation of an extract, the eyestalks are 

 homogenized, boiled, and centrifuged, and then 

 the supernatant is removed for use. In this way, 

 an investigator obtains a protein-free extract, 

 the chemical composition of which is in other 

 respects unknown. 



Several workers have attempted to demon- 

 strate a direct effect of crustacean molt-inhibit- 

 ing hormone on tissue respiration by the addi- 

 tion of such eyestalk extracts to homogenates. 

 Scheer, Schwabe, and Scheer (1952) have re- 

 ported that, in general, homogenates of mus- 

 cle from eyestalkless individuals of the green 



crab (Carcinus maenas), the lobster (Homarus 

 gammarus), and the shrimp (Palaemon squilla) 

 respire at a lower rate than do homogenates of 

 muscle from unoperated individuals, and that 

 the rate increases after the addition of eyestalk 

 extract to the homogenates. However, the data 

 submitted by these authors show that the effects 

 are rather small and quite variable. Further- 

 more, their data indicate that the addition of 

 eyestalk extracts to homogenates of muscle 

 from Palaemon squilla in some cases increases 

 and in others decreases the respiratory rate. 

 As noted above, crustaceans and their excised 

 tissues respire at different rates according to 

 stage in the molt cycle. Variability in results, 

 therefore, may be related to stage. Schwabe, 

 Scheer, and Scheer (1952) consider that synthe- 

 sis of the new exoskeleton begins during the late 

 intermolt period, that is, in late stage C. This 

 concept does not agree with that of Drach (1939), 

 according to whom the synthesis of a new exo- 

 skeleton begins during the early premolt period, 

 that is, in early stage D. If Drach's criteria, 

 which are accepted by the majority of workers 

 (see Renaud, 1949; Travis, 1955; Charniaux- 

 Cotton, 1957; Skinner, 1958, [MS]; Passano, 

 1960), are valid, the variability in the results of 

 Scheer, Schwabe, and Scheer (1952) may be 

 attributed to the fact that some of their animals 

 were in the intermolt stage and some were in the 

 premolt stage. 



For several species of crustaceans, Kuntz 

 (1946) noted that low concentrations of sinus 

 gland extract increased the rate of reduction of 

 methylene blue by midgut gland and that high 

 concentrations decreased the rate. A more com- 

 plete report of this work has not appeared. 



In a series of experiments on Carcinus 

 maenas (Skinner and Bliss, unpublished data), 

 we found that homogenates of midgut gland con- 

 taining extracts of one to five sinus glands re- 

 duced methylene blue at essentially equivalent 

 rates. On the other hand, homogenates that con- 

 tain leg muscle equal in wet weight to two sinus 

 glands carry out this reduction one and one-half 

 times faster. 



The variability of results in experiments of 

 this kind emphasizes the need for the use of 

 (1) more highly purified hormonal preparations, 

 and (2) more definitive systems, such as those 

 containing isolated mitochondria or submito- 

 chondrial particles. 



