33 



The Development of the Individual 



Embryology 



The bodies of the more advanced animals are composed literally of 

 billions of cells, showing a great variety of shapes, sizes, and functions. 

 Yet, each of these animals started its life as a single cell, zygote, and the 

 adult has been produced by repeated duplications of this cell together 

 with the proper differentiation and integration of the cells produced. 

 In this chapter we shall learn something about the origin of the dif- 

 ferent tissues and organs, primarily in the chordates. This is not an 

 easy study, because many complex reactions take place as the genes of 

 the cells exert their influence and bring about an orderly arrangement 

 of the mass of protoplasm which appears during embryonic growth. 

 Also, there are many phases of this development which are not yet fully 

 understood ; but our understanding is increasing rapidly, especially in 

 the light of recent physiological experiments in this field. 



There are similarities in the embryonic development of the different 

 classes of chordates, but there are also differences due primarily to the 

 amount and distribution of yolk in the zygote and to the conditions 

 under which the embryo develops. We can understand the entire group 

 better, perhaps, if we first study the embryogenesis of a simple chordate, 

 such as Amphioxus, and then show how other chordates differ from this 

 form. 



Early Cleavage 



After the egg is fertilized (see Chapter 31) a zygote results which 

 usually undergoes its first mitosis within about an hour. The two cells 

 which are formed adhere to one another and a second division follows 

 shortly. Since both cells usually divide at about the same time, we now 

 have a group of four cells. This is followed by a third division of 

 each of these cells to give eight. Other divisions follow and give 16, 

 32, and 64 cells. At about this time most embryos lose their synchroniza- 

 tion of cell division and there is no longer a doubling of cells at regular 

 intervals. When the embryo is at about the 16-cell stage, the cells are 



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