142 FUNDAMENTALS OF STRUCTURE AND FUNCTION 



in a plane in the middle, or equator, of the spindle, this being 

 known as the meta phase. Next the half or split chromosomes 

 appear to be pulled apart by the spindle fibers so that an equal 

 number move toward each pole, where they come to rest. These 

 changes are called the anaphase. 



Here the spindle fibers which extended from one pole to the 

 other begin to thicken at the equator. The swellings grow larger, 

 fuse, and spread out to form a delicate plate, which eventually extends 

 clear across the mother cell. This cell plate is in the nature of a 

 plasma membrane which splits into two, forming the new cell wall 

 between the two new cells. The fibers of the spindle now disappear 

 and cell division is completed. Meantime the recently split chromo- 

 somes lose their identity and again take on the netlike appearance as 

 in the original resting cell. The last series of changes comprises 

 the telophase. 



How Animal Cells Divide 



The resting animal cell undergoes a similar process in division. 

 However, in the animal cell a new structure is found, called the 

 centrosphere, which is a small body lying in the cytoplasm near the 

 nucleus. A central granule, called the centrosome or centriole, is 

 found within this centrosphere. The centriole usually divides to 

 form two of these granules at the beginning of mitosis. The initial 

 stages of cell division, collectively called the prophase, occur when the 

 particles of chromatin scattered throughout the nucleus take the form 

 of the spireme or tangled thread. This thread thickens and shortens 

 and then breaks up into the individual chromosomes. The number 

 of chromosomes for the body cells of the individual of a species is always 

 constant. Among plants, for example, in the pea there are always 

 14, in the onion 16, and in the lily 24 ; while examples taken at 

 random among animals show 4 for certain roundworms, 8 for the fruit 

 fly, Drosophila, of which you will hear more later, 32 in one of the 

 common earthworms, 200 in one of the crayfishes, 24 Mn a common 

 locust, 24 in one of the frogs, and 48 in man. 



During the formation of the spireme the threads of the future spindle 

 are growing out from radiations, called asters, which appear around the 

 centrioles. (See figure on page 143.) As the process continues the two 



' This is not quite exact, for it has been found that in some animals at the time when the chromo- 

 somes are reduced in number in the process of maturation (see page 429) , there is an even number 

 in the female sex cells but an odd number in the male sex cells or vice versa. 



