On the development of the vascular system in the foetus of vertebrated animals. Part II. / [Allen Thomson].

  • Thomson, Allen, 1809-1884
Date:
1831]
Catalogue details

Licence: Public Domain Mark

Credit: On the development of the vascular system in the foetus of vertebrated animals. Part II. / [Allen Thomson]. Source: Wellcome Collection.

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    s. or perfected soonest, yet before the commencement of their for¬ mation, and certainly before any parts, which may with justice be compared, in function or structure, to these organs, are pro¬ duced, the substance is deposited from which the head, trunk, and extremities are form.ed. The heart has also been supposed to take its origin in conse¬ quence of some influence derived from the brain and spinal cord ; and many have imagined that the development of the greater number of organs in the body, follows necessarily the presence of the heart, or of certain bloodvessels; but the phe¬ nomena of the development of these parts appear to prove such ideas to be erroneous, and to show that we are as yet ignorant of any particular influence which the pre-existence of one part in the early stages of its advancement exerts on the formation of another. Many observations, indeed, show that the brain and heart are nearly simultaneously produced, and that in all those parts which may be examined with ease, a certain quantity of their parenchyma is formed, before they receive the bloodvessels which, at a later period, serve for their nutrition. IV. The general resemblance which the changes of de¬ velopment in the ova or foetuses of vertebrated animals bear to one another, is very striking ; it illustrates the analogy of struc¬ ture in the different animals of this class when arrived at their state of maturity, and seems to indicate very clearly, that the general plan upon which their systems and organs are construct¬ ed and arranged is the same. This correspondence, indeed, in the relations of organs to one another, and similarity in their construction, to which the name of Type of Organization has been given, appears to be comparatively much more clearly un¬ derstood, from the knowledge, confined as it yet is, of the de¬ velopment and transformation of the foetus, than from the immense number of facts which have been established by the examination of the structure of animals in their perfect state. In examining the embryoes of the vertebrated animals, at a corresponding period in the early stages of their development, it is truly surprising to remark how very much they resemble one another. In some of the higher orders especially, it would be difficult for those unaccustomed to such investigations, to distinguish between the embryoes of the lizard, bird, or mammi-