xm] SIZE AND SHAPE 199 



persists from one mitosis to the next is not the precise 

 masses of chromatin which constitute mitotic chromosomes, 

 or at least make up the greater part of their substance, but 

 rather a framework or basis in which a greater or less amount 

 of chromatin is concentrated at different times. Here again 

 the analogy though it is only an analogy of independent 

 symbiotic organisms may be helpful. If such organisms, for 

 example 7 J ooxanthellae ', contained chlorophyll, the amount 

 of it might vary from time to time, giving rise to corre- 

 sponding variation in their visibility, but the organisms 

 would nevertheless persist as individuals even though at 

 times they were only distinguishable with difficulty in con- 

 sequence of their temporary poverty in chlorophyll. The 

 chromosomes, in fact, must not be imagined as simple 

 masses of chromatin, but as organised bodies which contain 

 a greater or less amount of chromatin at different times, 

 and it is the bodies themselves that must be regarded as 

 persistent individuals, and not merely the chromatin which 

 by its staining capacity makes them especially visible 

 during mitosis. 



(3) The recognisable and definite differences in size and 

 shape of the chromosomes. 



Under the heading of the definite number of chromo- 

 somes no explicit reference was made to the constant diffe- 

 rences of size and shape which characterise the chromosomes 

 of many animals, although this was implicitly assumed in 

 referring to WILSON'S observations in Metapodius. In quite 

 a large number of animals, especially in the Orthoptera, 

 Hemiptera and Diptera among insects, and in Echinoderms, 

 some or all of the chromosomes are constantly recognisable 

 by their size, by their shape in the anaphase of mitosis, and 

 frequently also by the shape of the bivalents in the first 

 maturation (heterotype) division of the germ-cells. In 



