548 OF NUTRITION. 



condition, both of form and structure, so that the part in which this change 

 takes place becomes fitted for some special function, and is advanced towards 

 the state in which it exists in the highest or most completed form of its specific 

 type. Thus the development of tissue consists in the change from a simple mass 

 of cells or fibres into any other form ; as in the production of dentine from the 

 cellular substance of the tooth-pulp ( 279), or in the formation of bone 

 in the sub-periosteal membrane ( 267). So, again, the developmental 

 change is seen in the passage of an entire organ from a lower to a higher con- 

 dition, by the evolution of new parts, or by a change in the 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-ex- 

 isting. Now it is in the early period of embryonic life, that the develop- 

 mental process is most remarkably displayed ; for it is then that we see that trans- 

 formation of the primordial cells into tissues of various kinds, which originates 

 a special nisus in each part, whereby the production of the same tissue in con- 

 tinuity 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 de- 

 velopment of tissue in particular directions, which makes all subsequent evolu- 

 tion but a completion or filling up of the plan thus sketched out. Thus during 

 the first few days of incubation in the Chick, the foundation is laid of the verte- 

 bral column, the nervous centres, the organs of sense, the heart and circulating 

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

 and of 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 completer evolution of the generative 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. Aad when the adult type has been once completely at- 

 tained, every subsequent change is one rather of degeneration than of develop- 

 ment, of retrogression rather than of advance. 



588. The difference between the two processes of Growth and Development 

 is most characteristically shown in those cases in which there is a partial or 

 complete arrest of one of them without any corresponding impairment of the 

 other. Thus a dwarf, however small in stature, may present a perfect develop- 

 ment 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 dimen- 

 sions, and yet some important part may be found in a condition of arrested 

 development ; thus the heart may consist of but a single cavity, or the inter- 

 ventricular or interauricular septa may be incomplete, so that it 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 



