Through What Stages Do Different Kinds of Organisms Pass? 



Similarities in Development At the very start, every animal is like a 

 protozoon; it exists as a single cell. In a large number of more complex 

 animals, like the starfish, the snail, the lancelet, there is a stage m the 

 development that consists of a hollow sphere of cells. In the development 

 of the frog, birds, and many other animals this hollow-sphere stage is not 

 so clear, being obscured by the yolk. The hollow sphere caves in and the 

 opposite sides meet, forming a two layered cup. This stage of the organism 

 may be compared to such animals as the hydra, which never gets much 

 farther than being a two-layered cup (see illustration, p. 274). Then the 

 two-layered cup becomes longer, suggesting certain kinds of worms. 



The embryos of animals that are closely related, such as several kinds of 

 backboned animals or several kinds of insects, show still more remarkable 

 resemblances. Thus the fish, the bird, the salamander and the rabbit con- 

 tinue very much alike when it is aheady possible to distinguish head and 

 trunk and limbs (see illustration, p. 459). In a somewhat later stage it is 

 not difficult to distinguish the bird from the fish or the tortoise. But at this 

 stage there are still certain resemblances between the birds and the reptiles. 

 Moreover, the embryos of several mammals (rabbit, pig, sheep and man, 

 for example) are at this stage strikingly similar. As they become older, 

 they become more and more different. 



Metamorphosis in Man^ In general form, the human infant resembles 

 the adult. We therefore do not commonly think of metamorphosis in 

 human beings. But if we compare the proportions of a baby with the pro- 

 portions of an adult, we can see that the changes are real. But a man is 

 something more than a large baby, something different in every detail (see 



illustration, p. 347). 



We know, of course, that as we become older many changes take place 

 in the proportions of the various external organs, particularly of the head 

 and face. Changes take place also in all the internal organs, in the relative 

 sizes of the heart and lungs and liver and stomach. Some organs that are 

 present in infancy may disappear. Others not present at one stage make their 

 first appearance later on. Some structures which appear at first to be form- 

 less knobs or buds gradually acquire definite shapes, with distinct parts, as 

 the body reaches maturity. 



Like other animals, the individual human being develops from a single 

 and comparatively simple cell to a very complex being made up of many dif- 

 ferent organs. The organs, as we know, consist of many different tissues, 

 each consisting of coundess cells of distinct kinds (see page 348). Through a 

 series of cell divisions that double the number of cells at short intervals, the 



iSee No. 7, p. 365. 

 354 



