456 Organisms Are Products of 



more human beings; horses produce 

 horses; roses produce more roses. Living 

 things are of the same kind as their par- 

 ents. That seems natural to you. Our 

 problem is to see exactly why they are 

 like their parents. 



What is contained in egg and sperm? 

 You know that the sperm cell is a tiny 

 mass of cytoplasm with a nucleus. An 

 egg cell, too, is cytoplasm with a nucleus 

 and usually a certain amount of food be- 

 sides. Now experiments have shown that 

 it is almost always only the nuclei of the 

 sperm and egg that are of importance in 

 making the offspring like the parents. 

 The nucleus differs from the rest of the 

 living matter within cells in that it con- 

 tains chromatin in the form either of a 

 network or of scattered granules. Chro- 

 matin is a living substance that can be 

 stained readily with certain dyes. For 

 many years it was suspected that chro- 

 matin was important in causing the simi- 

 larity between parents and offspring. 

 But nothing could be seen inside the 

 chromatin to explain why this should 

 be so. 



The gene theory. A little over twenty- 

 five years ago a theory was advanced 

 that chromatin is composed of parts 

 called genes (jeans) which cause the 

 various characteristics to appear in the 

 organism. Professor Thomas Hunt Mor- 

 gan, who died in 1945, '^^^ ^^^ author of 

 the gene theory. He and his associates 

 and hundreds of biologists in many parts 

 of the world have been testing the 

 theory for more than twenty-five years. 

 They have much evidence that it is the 

 genes within the chromatin that cause 

 the various characteristics of the organ- 

 ism to appear. 



Heredity a?id Eiivirofiment unit ix 



Fig. 404 Thomas Hunt Morgan, who devel- 

 oped the theory of the gejte. (keystone view 

 company) 



Genes are thought to be too small to 

 be seen with the microscope; we say 

 they are ultramicroscopic. Their chemi- 

 cal make-up is not known but we know 

 that they must be able to assimilate, 

 grow, and form more of their own kind 

 of substance. 



Behavior of chromatin. Since chroma- 

 tin consisting of genes is the important 

 substance which is found in the gametes, 

 it should be worth our while to watch 

 its behavior as the fertilized egg cell goes 

 through cell division. Before the cell di- 

 vides, the chromatin seems to be strung 

 out in tiny granules or threads through- 

 out the nucleus. As the cell begins to 

 divide the chromatin material (gathers to- 

 gether in long, thin threads which pres- 

 ently become shorter and thicker. These 

 short pieces of chromatin are like rods, 

 some straight, some bent. They are called 



