56 EIGHTEENTH REPORT. 



The question then is, how can these separate lumps of chromatin, 

 each one perhaps the bearer of some separate trait, exert their in- 

 fluence upon the develoi)ment and functions of the cell? The bulk 

 of the nuclear network lies near the circumference of the nucleus, 

 it is true, but not all. One can hardly conceive that onlj- those 

 chromatin masses do function that lie near the circumference. 

 Even these it seems are not all in intimate contact with the cyto- 

 plasm but usually lie a short distance iuAvard from the nuclear 

 membrane. 



To explain the action of the chromatin upon the cytoplasm vari- 

 ous theories have been proposed. Perhaps the most i)opular of these 

 is that of enzymes. We know that there are enzymes that will 

 hydrolyze starches into sugars and others that change sugars into 

 alcohol and carbon dioxide; some enzj-mes facilitate oxidation, 

 others reduction ; some digest proteins, others dissolve cellulose or 

 the nearly allied pectose substances. These are however, all more 

 or less catalytic in their nature; they facilitate certain chemical 

 changes that would take place to a slight degree without their 

 aid. It is rather hard to connect any known enzymes with the 

 production of tlie i)eculiarities of morphology and physiology of 

 the various cells of a i)]ant or animal. 



There is, however, another class of little known substances which 

 exert profound effects upon the development of the higher animals. 

 These are the "hormones," the secretions of some of the ductless 

 glands of the bod3^ As yet, much that has been written about 

 them: is in sore need of critical review, but yet enough is known 

 to make certain of their existence and importance. 



Probably the best known of these substances are the secretions 

 of the thyroid gland, a ductless gland situated in the neck. If 

 this becomes atrophied during childhood further development 

 ceases, both physical and mental, and the child remains a dwarf, 

 with the mind of a child, no matter how old he may grow to be. 

 Let such a child be fed the extract of the thyroid gland of a calf, 

 for instance, and develoi)ment begins almost ,at once and normal 

 growth and maturity, physical and mental, result, provided the 

 thyroid treatment is continued. If it is finally discontinued various 

 pathological developments of the skin ensue and finallj^ a state of 

 mind bordering on imbecility results. This can be cured, however, 

 by resumption of the thyroid feeding. Another secretion of this 

 nature is that of the pituitary gland situated at the base of the 

 brain. In cases of hypertrophy of this organ there results the dis- 

 ease known as acromegaly. This is characterized by the elongation 



