324 Cell as Basis of Organic Activity 



Comparative Embryology. — All of the multicellular animals orig- 

 inate as single, fertilized cells, the zygotes. These immediately start 

 dividing, and after a series of stages the adult forms are attained (Fig. 

 84) . In this development there are several recognizable stages : the early 

 cleavage, the morula, the blastula, and the gastrula. The more closely 

 related the animals are, the longer their embryos resemble one an- 

 other. Among the vertebrates, the embryos of all forms bear close 

 resemblance to one another at first ; gradually the features characteristic 

 of the species appear. For example, the embryo of a fish in early stages 

 is very similar to that of man. As development proceeds, the fish shows 

 its divergence first, but the embryo of a cat or dog still is very much 

 like that of the human being. With many of the invertebrates, it has 

 been possible to determine the true relationships of many highly aber- 

 rant adults by study of the developmental stages. 



Von Baer noted these resemblances and formulated some general 

 laws of embryonic development : ( 1 ) characteristics specific to the 

 organism appear after the more generalized ones; (2) during its de- 

 velopment, the animal progressively changes in form from other related 

 animals; and (3) the embryonic stages more nearly resemble the 

 young stages of other animals lower in the phylogenetic scale rather 

 than the adults. In the development of the human being, for example, 

 gill slits, a notochord, and aortic arches develop only to be shifted to 

 other functions as growth proceeds. 



Vestigial Organs. — In the bodies of most animals are struc- 

 tures which seemingly have no adult function. In the human body, there 

 are approximately ninety such structures among which can be listed 

 the muscles that move the ears, the nictitating membrane of the eye, 

 the appendix, segmental abdominal muscles, caudal vertebrae, and wis- 

 dom teeth. Their presence can be accounted for only by realizing that 

 they are vestiges of organs which were useful in ancestral forms. They 

 are gradually disappearing, but in the meantime they remain as mute 

 evidence of the theory of evolution. 



Comparative Physiology. — Animals are not only structurally sim- 

 ilar, but they are also functionally similar. Thus it is possible to study 

 the physiology of a dog, or even a frog, and learn much about human 

 functioning. These physiological relationships are widely employed 

 in the practice of medicine. The action of drugs can be tested on rats 

 and the results applied to human use. Endocrine products such as thy- 

 roxin can be obtained from the glands of cattle and used for medica- 

 tion in man. 



