660 THE PHYSIOLOGY OF REPRODUCTION 



Minot has compared the growth of the body to a man building 

 a wall.^ ' ' He begins at first with great energy, full of vigour ; 

 the wall goes up rapidly ; and as the labour continues, fatigue 

 comes into play. Moreover, the wall grows higher, and it 

 takes more effort and time to carry the material up to the top 

 of the wall, and to continue to raise its height, and so, as the 

 wall grows higher and higher, it grows more slowly and ever 

 more slowly, because the obstacles to be overcome have in- 

 creased with the very height of the wall itself. So it seems with 

 the increase of the organism, and with the increase of our 

 development, the obstacles to our growth increase." According 

 to Minot, the explanation of this phenomenon must be sought 

 in the differentiation of the protoplasm, which goes on growing 

 with an ever -increasing complexity as the cells of the body 

 multiply. 



It has just been mentioned that every cell has assigned to 

 it a definite limit in size beyond which it cannot go. Boveri ^ 

 has enunciated the general law that the process of cell division 

 is regulated by the proportion of chromatin material to cyto- 

 plasm, and that it comes to a standstill when the ratio of the 

 mass of chromosomes to that of the cells in any given tissue or 

 organ reaches a certain definite point. Furthermore, it is 

 stated that the size of the cells in any given tissue after active 

 cell multiplication has ceased, bears a definite relation to the 

 original mass of chromatin contained in the fertilised egg.^ 

 Thus it is pointed out that the mesenchyme cells of the embryo 

 developed from the artificially fertilised sea-urchin's egg are 

 only half the size of those of the embryo which has been pro- 

 duced by normal fertilisation, for although the parthenogenetic 

 and normally fertilised eggs are equal in size at the commence- 

 ment of segmentation, the latter possess initially twice as much 

 nuclear substance as the former.* 



The fact that cell division ceases when the ratio of the mass 



' Minot, "The Problem of Age, Growth, and Death," Popular Science 

 Monthly, vol. Ixxi., 1907 ; reprinted London, 1908. 



' Boveri, Zellen-Studien, Part V., Jena, 1905. 



" Robertson, " On the Normal Rate of Growth of an Individual and its 

 Biochemical Significance," Arch.f. Entwick.-Mech., vols. xxv. and xxvi., 1908. 



* Driesch, " tJber das Mesenchym von unharmonisoh zusammengesetzten 

 Keimen der Echiniden," Arch.f. Entwick.-Mech., vol. xix., 1905. 



