Text-book of the embryology of man and mammals / by Oscar Hertwig ; translated from the third German edition by Edward L. Mark.
- Date:
- 1892
Licence: Public Domain Mark
Credit: Text-book of the embryology of man and mammals / by Oscar Hertwig ; translated from the third German edition by Edward L. Mark. Source: Wellcome Collection.
Provider: This material has been provided by the Royal College of Physicians of Edinburgh. The original may be consulted at the Royal College of Physicians of Edinburgh.
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![Amplubia, Fishes, etc. Only through the observation of such various objects has insight been acquired into many processes, which in their essence remained unintelligible to us from the study of the Chick alone. For it was thus that one first learned to distinguish the important from the accessory and unimportant, and to understand the laws of development in theii- generality. In this text-book, therefore, I shall not confine mj^self to a single object, such as the egg of the Hen or the Rabbit, but from more general comparative standpoints shall endeavour to present what, through extensive series of investigations, we have thus far recognised as the rule in regard to the real nature of the processes of fertilisa- tion and cleavage, the formation of the germ layers, etc. However, let no one expect a text-book of comparative Embryo- logy. The purpose and the problem is first of all to learn to com- prehend the development and the structure of the human body. What we know about that has been placed before ever3rthing else, and the embryology of the remaining Vertebrates has been cited, and, as it were, fully utilised, only in so far as was necessary for the purpose indicated. In the division of the embryological material proposed by us, ac- cording to the separate systems of organs, there is a long series of processes, with which the development begins, which do not permit of an arrangement, because at the beginning the fundaments of definite, afterwards differentiated organs, are not recognisable in the germ. Before there is any formation of organs, the egg is divided into numerous cells, and these then arrange themselves into a few larger complexes, which have been called the germ-layers, or the primitive organs of the embryo. Further, in the higher Verte- brates there are formed certain organs, which are useful only during embryonic life, and are subsequently lost—namely, the foetal mem- branes and foetal appendages. All of the processes of that nature we shall treat of connectedly, and by themselves. In accordance with this, we can divide our theme into two main sections, the first of which will deal with the initial processes of development and the embryonic membranes, the second with the oiigin of the separate systems of organs. In order to facilitate for the advanced a more tliorough study, and a penetration into embryological literature, a survey of the more important original worlcs is given at the close of the separate chapters. On the other hand, text-books of Embryo- logy may be mentioned in this place. [Compare also the larger monographic works cited at the end of tho book.]](https://iiif.wellcomecollection.org/image/b2198184x_0023.jp2/full/800%2C/0/default.jpg)
