Fine structure of egg envelopes and the activation changes of cortical alveoli in the river lamprey, Lampetra fluviatilis

Most eggs are surrounded by several prominent envelopes which have been given names depending on their origin, structure or chemical composition. As our present knowledge of these envelopes is very fragmentary, the results of attempts to homologize the different layers between different animal groups are still open to debate. The nomenclature in this field is quite confusing. According to Raven (1961) the.egg membranes may be divided into ‘primary egg membranes’, formed in the ovary by the egg cell, ‘secondary egg membranes’ formed in the ovary by the follicle epithelium, and ‘tertiary egg membranes’ formed in the genital ducts after ovulation. The egg envelopes in the river lamprey, as in fish, are supposed to be primary egg membranes, although there is no certainty on this point. At least three distinct layers can be distinguished in the egg envelope of this species. Common to two of them is the presence of radial striations, which justifies the name ‘zona radiata’.