A manual of physiology / by E.D. Mapother.

  • Mapother, Edward Dillon.
Date:
1882
Catalogue details

Licence: Public Domain Mark

Credit: A manual of physiology / by E.D. Mapother. Source: Wellcome Collection.

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

    27/694 (page 15)
    CHAPTER II. Sec. I.—man's place in the animal kingdom. If we examine the various forms of animal life from the simplest to the most complex, we shall easily observe that the organization becomes elaborated by almost imperceptible de- grees as we ascend the scale. Beginning at the lowest point, we find unicellular organisms, each formed by a microscopic mass of protoplasm, apparently homogeneous in structure (monera of Hseckel) ; the first step in developmental advancement is found in the differentiation of the superficial layer of the proto- plasmic mass into a denser membrane or cell-wall; then organs of locomotion appear, either temporary, as in the psendopodia ofthe radiolaria, or permanent, as in the vibratile cilia of some infusoria ; organs of reproduction are next developed, as in the nucleus and nucleolus of the infusoria. This scale of gradual development is traceable step by step till we arrive at the highest degree of functional specialization as met with in man. The progressive specialization of organs and of functions can be followed not only throughout the animal series, but even in the evolution of a single organism. If, for instance, we take man from his origin, we find him at first represented by a single cell, the ovum ; he represents in this phase a unicellular animal. This cell then segments, and the division repeats itself till a mass of cells are formed, all identical in size and shape, and the segmented ovum now resembles a rhizopod deprived of its pseudopodia. Soon a part of these cells are set apart from the others to form a denser layer, and this again forms three leaflets from which all the organs are found to be derived, and each of the stages through which man passes during the period of embryonic development corresponds pretty closely to the mature condition of one of the inferior members of the animal kingdom. The analogy is still more striking if, instead of comparing the different stages of the development of man to animals already fully formed, we compare them to different stages of the development of other animals; the resemblance will then be found very close indeed.