16 Heredity and Environment 



perceptible change is minute ; but the changes are innumerable and 

 unceasing and in the end they accomplish this miracle of trans- 

 forming the fertilized egg cell into the fish or frog or man a 

 thing which would be incredible were it not for the fact that it 

 has been seen by hundreds of observers and can be verified at 

 any time by those who will take the trouble to study the process 

 for themselves. 



3. Cell Division. After fertilization the first step in devel- 

 opment is the cleavage or division of the egg. This is in the 

 main like any typical cell division and since the details of this 

 process are of extraordinary interest in the study of the mechan- 

 ism of heredity and development it is desirable to give at once 

 a rather detailed account of the way in which the nucleus and 

 cell-body divide. 



a. Mitosis or Indirect Division of the Nucleus. It was once 

 supposed that both the nucleus and the cell-body divide by a 

 simple process of constriction or direct division, but it is now 

 known that the nucleus rarely divides in this manner, and that 

 the nuclei of germ cells never do so. On the contrary the nu- 

 cleus almost always divides by a complex process known as 

 mitosis or indirect division (Figs. 6 and 7). During this process 

 the chromatin granules of the "resting" nucleus become arranged 

 in lines like beads on a thread (Fig. 8) ; these threads, which are 

 called chromosomes, are at first long and slender and much 

 coiled, but afterwards they grow shorter, thicker and straighter 

 and it can then be seen that in each species of animal or plant 

 there is a definite and constant number of these threads or chro- 

 mosomes (Fig. 6, A-D) ; this number varies from 2 to 200 in dif- 

 ferent species of animals, the most usual number being some- 

 where between 10 and 30, but so far as is known each species has 

 a constant number of chromosomes in every cell of the body. 



The nucleolus and the nuclear membrane then disappear, the 

 chromosomes move into the equator of the cell forming the equa- 

 torial plate (Fig. 6, F) and each one splits lengthwise into two 



