Volume 2

Text-book of embryology / edited by Walter Heape.

Date:
1914-
Catalogue details

Licence: Attribution-NonCommercial 4.0 International (CC BY-NC 4.0)

Credit: Text-book of embryology / edited by Walter Heape. Source: Wellcome Collection.

Provider: This material has been provided by The University of Leeds Library. The original may be consulted at The University of Leeds Library.

    17/616
    SEaMENTATION, GASTRULATION, AND THE FORMATION OF THE QEEM LAYEES The Vertebrate begins its individual existence in the form of a single cell, the Zygote or fertilized egg, which in turn originates in the process of fertilization by the fusion or conjugation of two gametes.^ Of these the microgamete or spermatozoon, derived from the male parent, is of relatively insignificant bulk as compared with the macrogamete or unfertilized egg. As a consequence the more obvious features of the Zygote, such as shape, size, and so on, are simply taken over from the macrogamete—in other words, they are of maternal origin. Such maternal features may remain obvious for some time during early stages of development, so long in fact as the maternal protoplasm remains predominant in 'bulk as compared with that elaborated under the control of the Zygote nucleus, but it seems unnecessary to assume that this fact has the important bearing upon questions connected with Heredity which has been claimed for it by some workers on Invertebrates. The Zygote is a typical cell, composed, so far as its living substance is concerned, of cytoplasm and nucleus, the cytoplasm containing a lesser or greater amount of food-material or yolk. In shape it is in the vast majority of cases approximately spherical. In the Myxinoids it is elongated, almost sausage-shaped, and in a certain number of cases, for example Amia, its shape is literally oval. The macrogamete—and therefore the Zygote—differs much in •size in different Vertebrates, ranging from about -1 mm. in diameter in Amphioxus to as much as 85 mm., or more, in the case of the African Ostrich. In some of the Sharks the size of the Zygote is also very great and this was doubtless the case too, with tliat of such extinct birds as Aepyornis.^ Such relatively huge Zygotes are of ' A general account of tlie processes of gametogencsis and fertilization has already been given in Vol. I. and they arc not further dealt with in this volume. - Assuming that the Zygote of Aepyurids bore tlie same ratio in size to its protective envelopes as does that of the Ostrich it would measure about 160 mm. in diameter. VOL. II , „