n] THE NUCLEUS 15 



other animals. That it is not universally valid has now been 

 made quite clear, for measurements of the nuclei and cells 

 in different parts of one embryo may show easily recognised 

 differences in the ratio of nuclear to cytoplasmic volume, 

 even before any conspicuous differentiation into tissues has 

 appeared 1 . But that some such relation exists in cells 

 which are homologous, that is, of similar origin and function, 

 is shown by such observations as those of BOVERI (1903 a) 

 on nuclear and cell size in Sea-urchin embryos. A Sea-urchin 

 egg can be made to develop when unfertilised, and when it 

 does so its nucleus is only half the size of that in the fertilised 

 egg, since the latter is formed by the union of two nuclei, 

 derived respectively from the egg and spermatozoon (PI. I, 

 A, ). When the cells of young larvae derived from fer- 

 tilised and unfertilised eggs are compared, it is found that 

 in an area of given size there are twice as many cells in a 

 larva from an unfertilised egg as there are in a like area in 

 a larva from a fertilised egg. That is to say, in larvae from 

 unfertilised eggs both cells and nuclei are half as large and 

 twice as numerous as they are in those from fertilised eggs, 

 with the result that the volume relation between nucleus 

 and cytoplasm remains constant although the nuclei are 

 twice as large in the one case as in the other. 



HERTWIG was led to formulate his law of the karyo-plas- 

 matic or nucleo-cytoplasmic ratio from observations on the 

 growth and division of Protozoa. In these animals also it 

 has been shown that the law is not universally valid, for 

 under different conditions of temperature and other en- 

 vironmental circumstances the ratio may vary, but under 



1 This has been shown especially by CONKLIN in the endodermic cells of the 

 Mollusc Crepidula, in which he finds great differences in the Kernplasma-relation 

 indifferent blastomeres,and in the same blastomere at different stages, depending 

 largely on the rate of cell division, of which it is an effect rather than a cause. 



