matter. This is the growing up or development of the 

 young individual into the form of the parent. 



In the simplest forms this has been seen to be a rather 

 easy thing. In the case of bacteria, or Paramecium, when 

 the parent divides, each of the progeny has to double its 

 size. In the bacteria this requires only a half-hour or so 

 under favorable conditions. In Paramecium it takes a 

 little longer. Where the parent differs from the offspring 

 only in size the problem is merely one of growth. 



2. How About the Higher Organisms? When we come 

 to a higher plant or animal, in which there are numerous 

 different kinds of organs in the adult, we see that the 

 work of the embryo in growing up is very much 

 complicated. The one-celled fertilized egg has in it 

 absolutely none of the leaves and roots, or brain and 

 muscles and bones which the parent organism has. The 

 most marvelous thing of all to the biologist is that a cell 

 with nothing of any of these tissues or organs in it, with 

 only a little mass of protoplasm, can develop these 

 complex structures. The wisest biologist has no final 

 answer to this question, and yet every egg that develops 

 answers the question in its own practical way. Of course 

 the student sees that it is no answer to the question to say 

 that the egg inherits the power and the tendencies from 

 its parents. This only gives a name to the process. Where 

 and how do these powers and tendencies reside in the 

 embryo? 



Development is much more interesting and difficult in 

 the higher animals and plants because it means a great 

 deal more than merely growing. It means becoming 

 different. 



3. What is Development in Higher Forms? There are 

 three important elements in this mysterious, process, and 

 while we cannot explain why it takes place by describing 

 these steps, it does make the process seem somewhat 

 clearer to us. 



a. Growth. In the first place growth is important, as 

 it was in the lowly forms. Indeed the larger plants 

 and animals grow more in becoming mature than 

 the simpler ones. Often the increase is a million- 

 fold. The great red woods of California, which 

 are among our largest plants, and whales, which 

 are about as large animals as have ever lived, arise 

 from eggs that require the microscope to enable us 

 to see them. This increase in size is growth. 

 Growth is nothing but this. 



b. Cell Division. When a bacterium divides it makes 

 two bacteria, and thus the individual is kept small. 

 When the egg of a whale divides the daughter cells 

 remain together and we have a two-celled whale 

 embryo. These continue to divide, and since they 

 continue to remain together the embryo comes to 

 have more and more cells. This makes the 

 organism complex. As these cells grow it becomes 

 larger as well as more complex. 



c. Differentiation. Growth and cell division are not 

 easy to explain, but the most difficult thing about 

 development is differentiation. This means that 

 these cells, all derived from one parent cell, are not 

 alike. They may seem to start alike, but great- 

 granddaughters of the same cell may give rise to 

 cells as different as nerve cells, skin cells, bone 

 cells, blood cells, and many others. 



The young of the higher plants and animals have a long, 

 difficult, and wonderful road to travel before they come to 

 be like their parents. It is "natural" for them to do it, 

 and we fully expect it to happen, but it is a marvel, none 

 the less. ' 



4. Direct Development. The egg of no one of the 

 higher organisms looks in the least like the parent. If we 



take the chick, for example, the egg when fertilized has 

 no resemblance to the chicken. Yet when the chicken 

 hatches, in twenty-one days, the young chicken looks very 

 much like the adult. It still needs to grow, and to 

 differentiate a few things, like feathers, etc., before it is 

 just like the parent. Now if we go back in the egg to the 

 eighteenth or nineteenth day it still looked like a chick, 

 but a little less so. By studying it thus we can find that 

 it came very gradually to take on its likeness to the 

 parent. There are no sudden or sharp changes in its 

 development. We call this sort of development direct. 

 This is a very common method of development. It is 

 found in mammals, in birds, in reptiles, in most fishes, and 

 in many lower forms. 



5. Metamorphosis. Not all animals, however, develop 

 in this way. We have already seen in the frog that the 

 thing that hatches out of the egg does not look at all like 

 a frog. It must make more changes after hatching than 

 the chick does. This tadpole, which looks more like a fish 

 than like a frog, may grow as a tadpole and live an 

 independent life for quite a period before it begins the 

 changes that make it appear as a frog. We see it as three 

 different things: egg, tadpole, and frog. This indirect 

 development is called a metamorphosis. It differs from 

 the direct development in this: in a metamorphosis the 

 organism becomes like the adult by passing through one 

 or more somewhat sharp or sudden changes or stages. 

 Between these points of sudden change there are periods 

 during which there is little or no change but growth. 



Some of the most beautiful illustrations of metamor- 

 phosis in the growing up of offspring are found in the 

 insects. Very many of the insects undergo a profound and 

 striking metamorphosis. In the moths and butterflies we 

 have a story something like this: 



(1) There is considerable food in the egg, and when 

 it hatches the animal that comes out is a small 

 worm-like object which we call a caterpillar 

 (larva). 



(2) This feeds on leaves and the like and grows, but 

 remains a caterpillar for weeks or months, during 

 which time it stores up a large quantity of food. 



(3) At the close of this period the caterpillar becomes 

 quiet and inactive, often secreting about itself a 

 protecting case. 



(4) In this cocoon the whole structure is gradually 

 but profoundly changed within, though there is no 

 iifference in the outside appearance of the cocoon. 

 Old organs are taken up by the blood and new 

 ones, suitable to the mature life, are developed 

 from the stored-up material. Wings, of which 

 there was no trace in the larva, long legs instead 

 of the poor, stumpy ones, different eyes and mouth 

 parts appear. 



(5) When this has all happened the changed animal 

 breaks from the cocoon in the adult form of the 

 butterfly. In this form it reproduces by eggs and 

 sperms, and mating takes place. Thus the cycle 

 is complete. 



6. Possible Advantages in Metamorphosis. Without 

 doubt this round-about development has both its 

 advantages and disadvantages to the species, but on the 

 whole it looks as though it aids the process of successful 

 reproduction more than it hurts it. The adult butterflies 

 are adapted to taking nectar from flowers. But their 

 caterpillars, which are able to eat voraciously of the 

 leaves of early spring, can get a much more successful 

 and early start than the nectar-living adult could do. 

 Furthermore, the adults of all such forms, flies, butterflies, 

 beetles, etc., appear full grown. They could not grow 

 except by moulting all the complex outside covering and 



