Morphology - Introduction: (2) Cell Development a Process of Differentiation
2. In the course of its development every cell proceeds from a condition in which it closely resembles every other cell, through a series of stages of gradually increasing divergence, until it reaches that condition in which it presents the characteristics features of the elements of a special tissue. The development of the cell is therefore a gradual progress from the general to the special state.
The like holds good of the development of the body as a whole. However complicated one of the higher animals or plants may be, it begins its separate existence under the form of a nucleated cell. This, by division, becomes converted into an aggregate of nucleated cells: the parts of this aggregate, following different laws of growth and multiplication, give rise to the rudiments of the organs; and the parts of these rudiments again take on those modes of growth and multiplication and metamorphosis which are needful to convert the rudiment into the perfect structure.
The development of the organism as a whole, therefore, repeats in principle the development of the cell. It is a progress from a general to a special form, resulting from the gradual differentiation of the primitively similar morphological units of which the body is composed
Moreover , when the stages of development of two animals are compared, the number of these stages which are similar to the another is, as a general rule, proportional to the closeness of the resemblance of the adult forms; whence it follows that the more closely any two animals are allied in adult structure, the later are their embryonic conditions distinguishable. And this general rule holds for plants no less than for animals.
The board principle, that the form in which the more complex living things commence their development is always the same, was first expressed by Harvey in his famous aphorism, "Omne vivum ex ovo," which was intended simply as a morphological generalization, and in no wise implied the rejection of spontaneous generation, as it is commonly supposed to do. Moreover, Harveys study of the development of the chick led him to promulgate that theory of "epigenesis," in which the doctrine that development is a progress from the general to the special is implicitly contained.
Caspar F. Wolff furnished further, and indeed conclusive, proof of the truth of the theory of epigenesis; but, unfortunately, the authority of Haller and the speculations of Bonnet led science astray, and it was reserved for Von Baer to put the nature of the process of development in its true light, and to formulate it in his famous law.
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