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Abstract The vascular supply of the seed of Ceratozamia mexicana Brongn. consists of internal and external vascular systems. Four to six vascular bundles from the megasporophyll enter the seed through the hilum and form a plexus. Within the plexus, the bundles divide, anastomose, and form a coroniform arrangement. This corona is observable prior to ligni?cation of the sclerotesta. Vascular bundles emerge from the plexus and pass through the sclerotesta to form the internal and external vascular systems. The external system consists of 10 bundles and the inner system of up to 40 at the level where the nucellus becomes free from the integument. The term pachychalaza? is introduced for the description of gymnosperm seeds. These vascular systems are illustrated in detail. Finally, the implications of this vascular structure are discussed with respect to the phylogenetic origin of the integument.