FERTILISATION 227 



many times greater than in other parts of the animal. An investi- 

 gation of these gradients has furnished him with information which 

 has made possible the experimental control of morphogenesis and 

 the development of specific form in the Planarian, perhaps the most 

 remarkable triumph in the annals of recent biological research. 



In the Planarian, the study of these gradients shows that the 

 metabolic processes are most active in the head region and that they 

 diminish as we pass down the main axis of the worm, being lowest 

 in the tail region. Those portions of the worm having the highest 

 rate control those with a lower rate. If by transverse section we 

 cut off the head and tail of a Planarian, the frequency with which it 

 will regenerate a new head at one end x of the remaining portion of 

 the worm, will be in direct relation to the height of the metabolic 

 rate at this end x, and in inverse relation to the metabolic rate at 

 the other end of the piece y. If x is higher than y, then a head will 

 form at x, if y is higher than x, a head will form at y. If x and y 

 have about the same metabolic rate, then we will have even chances 

 that a head or tail will form at ./ or ;//. Now the metabolic rate at 

 x and y can be decreased or increased at will by certain reagents, or 

 functional activity, and so a head can be made to appear where, 

 under less stimulation, a tail would normally regenerate, and thus 

 the process of regeneration can be definitely controlled. 



The study of the unfertilised egg-cell shows that its metabolic 

 rate is low, and the evidence discussed in one of the foregoing 

 sections amply testifies to this, in the astoundingly small oxygen 

 consumption of the unfertilised egg. On fertilisatpn a considerable 

 increase of susceptibility takes place, which in Nereis reaches its 

 height when the free-swimming stage is attained, while in Arcnicola 

 it is only reached when the young worm has developed live or six 

 segments. In the sea-urchin Arbacia, and the starfish Axterins, the 

 metabolic rate as determined by the direct susceptibility test reaches 

 its highest level at the gastrula stage, and from this onwards slowly 

 decreases, rejuvenescence comes to an end and old age begins. In 

 Vertebrates much the same thing holds, in Fa ml it Hi* rejuvenescence 

 occurs during the early stages of development, but as soon as the 

 periblast forms and the embryo assumes its shape, senescence 

 commences. In the wrasse Tautogolalrus, the period of increasing 

 susceptibility continues up to the time of hatching, and so to a much 

 later stage than in Finidtilitx. In the. frog and the salamander the 

 average susceptibility increases from fertilisation onwards through 

 segmentation and gastrulation to the formation of the embryo and 

 somewhat beyond the hatching stage; after this senescence com- 

 mences. The animal life cycle is therefore a more or less brief 

 period of rejuvenescence followed by a longer and more gradual 



