Received for publication 9 May 1983. Use of a company or product name by the Department does not imply approval or recommendation of the product to the exclusion of others which also may be suitable. The authors wish to thank George A. Brown and David B. Root, biological laboratory technicians, and Mark Cherry, YACC intern, for their valuable assistance. The cost of publishing this paper was defrayed in part by the payment of page charges. Under postal regulations, this paper therefore must be hereby marked advertisement solely to indicate this fact.