702 THE PHYSIOLOGY OF REPRODUCTION 



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 increased 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 1 has 

 enunciated the general law that the process of cell division is 

 regulated by the proportion of chromatin material to cytoplasm, 

 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. 2 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 produced by normal fertilisation, for although 

 the parthenogenetic and normally fertilised eggs are equal in size at 

 the commencement of segmentation, the latter possess initially twice 

 as much nuclear substance as the former. 3 



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

 chromosomes in the nuclei of an egg (or of a tissue or organ) to that 

 of the surrounding protoplasm reaches a certain definite limit, is 

 regarded by Loeb 4 as evidence that this ratio is determined by the 

 laws of mass action and chemical equilibrium. He says further 

 that if this conclusion is correct the synthesis of nuclein com- 

 pounds, from their protoplasmic constituents, must be a reversible 

 process. 



The fertilisation of an ovum is immediately succeeded by an 

 enormous synthesis of nuclear material. In the cellular division 

 which follows, each new 'nucleus is of the same size as the parent 



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



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

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

 "Studies in the Growth of Man," Amer. Jour, of Phys., vol. xxxvii., 1913. 

 See also Feldraan, Principle* of Ante- Natal and Post-3 atal Child Physiology, 

 London, 1920. 



3 Driesch, " Uber das Mesenchym von unharmonisch zusammengesetzten 

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



4 Loeb, The Dynamics of Living Matter, New York, 1906. 



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