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 parevts. 
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, claws, 
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. 
