Vertebrate embryology : comprising the early history of the embryo and its foetal membranes / by J.W. Jenkinson.

  • Jenkinson, J. W. (John Wilfrid), 1871-1915.
Date:
1913
Catalogue details

Licence: In copyright

Credit: Vertebrate embryology : comprising the early history of the embryo and its foetal membranes / by J.W. Jenkinson. 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.

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    INTRODUCTION Embryology is the study of the development of the indi\ idual organism—that is to say, of that long and frequently complex series of changes whereby from a relatively simple germ there is produced a new individual which, within the limits of ordinary variation, is like the parents that give it birth. Development, in other words, is the production of the form characteristic of the species ; it is the mechanism of inheritance. The starting- point of the process may be a single cell, which is either a fer- tilized ovum or at any rate the product of the union of two cells, as in all cases of sexual reproduction, or else an unfertilized egg-cell, as in parthenogenesis. Again, the ‘ germ ; from which the new organism is to arise may be a multicellular body derived from one or more of the tissues of the parent, as in budding. Lastly, in regeneration, or the replacement of lost parts, where a new whole form is produced over again from a part of the original—whether that part is large or small—the process starts from a multicellular and a differentiated structure. As a matter of common practice, however, the term ‘ develop- ment ’ is often restricted to the first of these processes, and in the Vertebrates with the greater justification in that reproduction by budding does not occur in the group, although regeneration does. Neither has natural parthenogenesis ever been observed. Reproduction, then, in Vertebrates means sexual reproduction, and the developing individual springs from the union of two germ-cells. These germ-cells arc the vehicles whereby the inheritable characters of the species are handed on from one generation to the next; they arc the material basis of inheritance. In the study of this process two methods are at our disposal. Either we may content ourselves with a description of the series of changes which the ovum passes through, or else we may add