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Analysis of HCB residues in the parts per billion range in cereals is hampered by interference from grain lipids. In fatty foods, analysis using solvent countercurrent extraction is made difficult by unfavorable distribution. HCB is not readily separated from other pesticides by GLC. These difficulties are discussed and partly resolved. Procedures are suggested for the extraction, steam distillation, and alkaline degradation of HCB as possible solutions to the analysis of HCB residues.

Both acetyl- and pseudocholinesterases have been detected in the atrioventricular specialized tissue of the human fetus. The upper part of the node, derived from sinus horn musculature, is cholinesterase-negative but contains acetylcholinesterase-positive nerves. The lower portion of the node and the common trunk of the bundle differentiate from atrioventricular canal muscle, are intrinsically pseudocholinesterase-positive and do not contain nerves up to the 190-mm crown-rump length stage. These observations are compared with those of previous workers who believed the specialized tissue of adult specimens of several species to contain only acetylcholinesterase. The possibility of a developmental change in the type of cholinesterase in the specialized tissue is discussed in relation to a recent study of the atrioventricular node of the neonatal rat. It is believed that the atnioventnicular node is a compound structure which may be considered in two portions having diverse developmental origins (3, 20). There is an upper part (Fig. 1) derived from muscle of the sinus horn and a lower component formed from atrioventricular (AV) canal musculature. At the 40-mm crown-rump length stage, the upper part of the node and the AV canal musculature are recognizable as separate structures. However, in a fetus of 190-mm crown-rump length the lower portion of the node or bundle extension is seen to have pierced the fibrous annulus and runs under and in contact with the upper part of the atnioventnicular node. Recent studies have shown that cholinesterases of both true and pseudo types are present in the heart and other organs from human fetuses of 28-mm crown-rump length and upward (21). This knowledge has stimulated histochemical evaluation of the conducting tissue at much earlier stages of human development (28-190-mm crown-rump length) than those considered by Navaratnam (17) (250 to term).