AGE, GROWTH, AND DEATH 



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tions the cells appear very much alike. The nuclei are 

 all similar in character, and for each of them there is 

 more or less protoplasm ; but the protoplasm in all 

 parts of these young rabbits is found to be very 



similar ; and indeed if we 

 should pick out one of these 

 cells and place it by itself un- 

 der the microscope, it would 

 be impossible to tell what part 

 of the rabbit embryo it had 

 been taken from, so much do 

 all the cells of all the parts 

 resemble one another. We 

 learn from this picture that the 

 embryonic cells are all very 

 much alike, simple in charac- 

 ter, have relatively large nu- 

 clei, and only a moderate 

 amount of protoplasm for 

 each nucleus to complete the 

 cell. 



Very different is the con- 

 dition of affairs which we find 

 when we turn to the micro- 

 scopic examination of the 

 adult. Did time permit it 

 would be possible to study 

 a succession of stages and 



FIG. 10. PORTION OF A 

 TRANSVERSE SECTION OF THE 

 SPINAL CORD OF A HUMAN EM- 

 BRYO OF FOUR MILLIMETRES. 

 Harvard Embryological Collec- 

 tion, series 714. The spinal cord 

 at this stage is a tubular struc- 

 ture. The figure shows a por- 

 tion of the wall of the tube ; the 

 left-hand boundary of the figure 

 corresponds to the inner surface 

 of the tube. 



show you that the condition which we are about to 

 study as existing actually in the adult is the result of a 



