HOW CHARACTERS ARE TRANSMITTED 129 



taken place, producing different parts of many kinds, but they 

 are of the same kind and in the same position as in the parents. 



In a few cases accidents happen and development does not 

 proceed in the orderly manner that commonly characterizes 

 reproduction. These cases are extremely rare, but of sufficient 

 interest to constitute the material of a separate chapter, dealing 

 with what happens when development goes wrong. 



Termination to growth. Still another marvel attends upon 

 growth and differentiation, and that is, that it should all stop at 

 the right point. It is difficult to comprehend that a man's arm 

 should grow at all so as to be an arm and not a leg, but once 

 started it is still more difficult to understand what should stop 

 the growth at exactly or approximately the right time, and not 

 allow it to proceed indefinitely, as it does in the nails, clawSj 

 and, to some extent, in the teeth of animals. 



In general, plants have no "typical termination" to their 

 growth, but increase in size as long as life lasts ; that is, they 

 seem unable to discharge their function except in connection 

 with new growth and by means of recently formed tissues, while 

 animals " get their growth," that is, function independently of 

 new growth. 



Summary. The only possibility of transmission of the unit characters of 

 the parents to the offspring is by means of the minute bits of matter con- 

 tained in the single sex cell from each parent, because it is the only material 

 handed down to the new individual. 



How this microscopic bit of matter can contain all the potentialities of 

 the race and be able not only to grow and to differentiate with growth, but 

 to stop at the right point, — how it can do all this is a mystery, but the fact 

 is doubtless connected with the definite " architecture " or structure of the 

 germ plasm which contains always a definite number of chromosomes. 



Exercises. 1. Study the formation of pollen and the location of the pistil 

 together with the method of getting the pollen upon the stigma in a variety 

 of plants. Oats, wheat, beans, sweet peas, and hollyhocks are especially 

 recommended. 



2. Examine frog spawn, if any is available, and, if possible, obtain 

 mounted slides showing the early stages of embryological development. 



3. Set a nest of eggs under a hen and break one every other day after 

 the first week. 



