The development of the caudal region of the neural tube is compared in tailed mammals with that of the chick and human. In rat, mouse, opossum and pig, the lumen of the cord extends caudally in an even manner, whereas in the chick and in man the addition of small cavities to the lumen results in a phase of irregular growth. In mammals with unreduced tails, the site of closure of the posterior neuropore is at the tip of the tail, whereas in pig, man and in the chick closure occurs before the

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... ation of the tail-bud. The teratological implications of these findings are discussed.