FORMATIVE POWER OF INDIVIDUAL PARTS. 417 



relations of those already existing, even though the change in its texture 

 should consist of little else than of simple increase: thus in the development 

 of the heart, we have the original single cavity subdivided, first into two, 

 and at last into four chambers; and in the development of the Brain, we 

 find the sensory ganglia to be the parts first formed, the anterior lobes of the 

 cerebrum to be evolved (as it were) from these, the middle lobes sprouting 

 forth from the back of the anterior, and the posterior from the back of the 

 middle; yet with all this, there is no production of any new kind of tissue, 

 the new parts being generated at the expense of histological components 

 identical with those of the pre-existing. Now it is in the early period of 

 emhrvmi'f life, that the developmental process is most remarkably displayed; 

 for it is then that we see that transformation of the primordial cells into 

 tissues of various kinds, which originates a special nisus in each part, where- 

 by the production of the same tissue, in continuity with that first formed, 

 comes to be a simple act of growth ; and it is then also that we observe that 

 marking out of all the principal organs, by the development of tissue in par- 

 ticular directions, which makes all subsequent evolution but a completion 

 or filling up of the plan thus sketched out. Thus, during the first days of 

 incubation in the Chick, the foundation is laid of the vertebral column, the 

 nervous centres, the organs of sense, the heart and circulating system, the 

 alimentary canal, the respiratory apparatus, the liver, the kidneys, and 

 many other parts; and at the termination of that period, the chick emerges 

 in such a state of completeness of development, that little else than increase 

 is wanting, save in the plumage and sexual organs, to raise it to its perfect 

 type. The same may be said of the Human organism ; save that the period 

 of its development is relatively longer, in accordance with the higher grade 

 which it is ultimately to attain; its earliest stages being passed through, 

 however, with extraordinary rapidity. The complete evolution of the gen- 

 erative organs, of the osseous skeleton, and of the teeth, constitute the 

 principal developmental changes which the Human organism undergoes in 

 its progress from the infantile to the adult condition ; almost every other 

 alteration consisting in simple increase of its several component tissues and 

 organs, without any essential change in their form or structure. And when 

 the adult type has been once completely attained, every subsequent change 

 is one rather of degeneration than of development, of retrogression rather 

 than of advance. 



333. The difference between these two processes of Growth and Develop- 

 ment is most characteristically shown in those cases in which there is a par- 

 tial or complete arrest of one of them, without any corresponding impair- 

 ment of the other. Thus a dwarf, however small in stature, may present a 

 perfect development of every part that is characteristic of the complete 

 human organism ; the deficiency being solely in the capacity for growth. 

 On the other hand, the usual size at birth may be attained, and every organ 

 may present its ordinary dimensions, and yet some important part may be 

 found in a condition of arrested development: thus the Heart may consist of 

 a single cavity, or the iuterventricular or interauricular septa may be in- 

 complete, so that the organ has not passed beyond the grade of development 

 which it had attained at an early period of embryonic life, although its 

 growth may have continued ; or the Brain may in like manner exhibit a 

 deficiency of the posterior lobes, or of the corpus callosum, or of some other 

 part whose formation nominally takes place in the latter months of intra- 

 uterine life, although the parts already produced may have continued to 

 grow at their usual rate. Numerous instances of the same kind might be 

 cited, but these must suffice. 



334. The demand for nutrition arises, however, not merely from the exer- 



