CHAPTER IX 



EMBRYOLOGY 



The discovery of this biological development of complex 

 living bodies from the single germ cell soon demanded the 

 attention of other sciences; and embryology especially con- 

 cerns the manner in which, at different stages, the growing 

 embryo still incomplete, simulates the general structure of 

 completed creatures of earlier species, and of less complex 

 form. 



It was noticed that when the development of the egg of 

 a fowl, was interrupted and examined at a very early stage 

 where the original germ cell had divided only a few times, 

 the embryo resulting might be compared with certain living 

 things whose life history goes no farther. Thus there is a 

 stage in the chicks' growth, when the original germ has 

 segmented until it has produced a bell-shaped globe open at 

 one end, having outer cells of one kind and inner cells of 

 another kind. This is the description of the complete 

 creatures of the early metazoa, and the similar metazoan 

 structure is arrived at in a similar way. 



To express it in other words it was found that when a 

 simple metazoan, or any similar creature whose organs are 

 still primitive, reaches its early maturity, it has gone through 

 changes, in subdivision and growth of cells, arising from 



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