108 PLAIN FACTS FOR OLD AND YOUNG 



the other becomes the taih Even man has a caudal 

 appendage at an early stage of his existence. After 

 a further laj^se of time, little excrescences, buds, or 

 ''pads" appear in the proper positions to represent 

 the arms and legs. After further development, the 

 ends split up into fingers and toes, and by the continued 

 development of the parts, perfect arms and legs are 

 formed. 



Simplicity of Early Structures.— The first struc- 

 tures formed are exceedingly simple in form. It is only 

 by slow degrees that the great complicity which char- 

 acterizes many organs is finally attained. For ex- 

 ample, the heart is at first only a straight tube. By 

 enlargement and the formation of longitudinal and 

 transverse partitions, the fully developed organ is 

 finally produced. The stomach and intestines are also 

 at first but a simple straight tube. The stomach and 

 large intestine are formed by dilatation; and by growth 

 of the tube in length while the ends are confined, the 

 small intestines are formed. The other internal organs 

 are successively developed by similar processes. 



The Stages of Growth.— At first, insignificant in 

 size, a simj^le cell, the embryonic human being steadily 

 increases in size, gradually apj)roximating more and 

 more closely to the human form, until, at the end of 

 about nine calendar months, or ten lunar months, the 

 new individual is prepared to enter the world, and 

 begin a more independent course of life. The follow- 

 ing condensation of a summary quoted by Dr. Austin 

 Flint, Jr., will give an idea of the size of the devel- 

 oping being at different periods, and the rate of prog- 

 ress: 



At the end of the third week, the embryon is a little 

 less than one-fourth of an inch in length. 



