HEREDITY AND VAKIATION 89 



purpose, is useful. So in all the higher organisms 

 these little masses of protoplasm, called corpuscles or 

 -cells, do not separate but hold together, some per- 

 forming one function, some another, and grow up 

 into a plant or an animal ; inter-cellular spaces being 

 left, which act as conduits for air or for excretory 

 products. 



The first change which took place was a physio- 

 logical one, and resulted in the separation of a 

 denser nuclear-plasm from a more watery substance 

 known as cytoplasm. The nuclear-plasm collected 

 into a nucleus, surrounded by the cytoplasm, which 

 protects it from external irritants. 



At a later stage nuclear-plasm again differentiated 

 into a nuclear-sap, formed of Achromatin, and a fine 

 net-work of threads formed of a substance called 

 Linin. Among these threads lie a number of 

 Chromosomes granules or threads made of Chro- 

 matin. And now, in many cases, during cell- 

 division, these different parts of the nucleus go 

 through a series of complicated processes, called 

 mitosis. Two minute bodies of Achromatin, called 

 Centrosomes, appear in the cytoplasm of the cell, at 

 opposite ends of the nucleus, from whence they send 

 out radiating arms, which appear to direct the move- 

 ments of the other parts ; arranging them, splitting 

 them, and finally separating them into two portions. 

 These star-like bodies are called Astrospheres. 17 



While the physiological variations were taking 

 place, a parallel series of morphological changes was 



17 The Centrosomes, together with the Astrospheres, are 

 called Centrospheres 



