DIFFERENTIATION AND REJUVENATION 159 



there was no brain, there was no spinal cord, nothing 

 that could possibly be called skin or muscle, or intes- 

 tine or heart. None of those things were yet produced. 

 But at sixteen and one half days in other words, after 

 a very brief period indeed only nine days of the whole 

 life of the animal there have arisen from this inchoate 

 beginning all the principal organs of the body. The 

 brain is there, divided up into its principal funda- 

 mental parts; the spinal cord has its nerves in con- 

 nection with the various parts of the body ; there is a 

 trace of the skeletal element ; the stomach, the liver, 

 the pancreas, the intestines, are all present and well 

 defined; the heart is a large and beating organ, amply 

 supplied with blood, connected with vessels, which 

 carry out and bring back the blood and are all far 

 along in their development. Equally instructive is the 

 microscopic examination, for we can see that the cells 

 themselves have been changed. Not only have the 

 great organs been mapped out in this brief period, but 

 the cells which belong to them have for each organ ac- 

 quired a characteristic quality. In the brain there 

 are nerve cells with their long processes to carry the 

 impulse in ; the single process (axon) to carry it out. 

 The glands in the stomach have the cells which are 

 to build them already there. The muscles which are 

 to move the stomach are beginning to appear as cells 

 of a special form. Nerve fibres extend down into the 

 gastric region and to the various distant organs of 

 the body. Muscle fibres can be recognised along the 

 back and in the limbs, and so in every part of the body 



