66 INTRODUCTION TO EVOLUTION 



system of arteries supplying blood to the head, another persists on the left 

 side as the connection between the heart and the dorsal aorta, and the 

 third forms the pulmonary arteries conveying blood to the lungs. Thus we 

 see how arteries "designed" for one purpose are made over to serve 

 other functions when the original function is discarded or becomes ob- 

 solete. 



Our account of human embryology has brought us through only the first 

 of the 9 months of prenatal development (Fig. 4.10). As we might antici- 

 pate, however, it is during this early period that the human embryo ex- 

 hibits the greatest number of characteristics comparable to those of 

 embryos of lower animals. Many more such examples of recapitulation 

 might be included in our account, involving such diverse organs as kid- 

 neys; brains; the skeletal bars, which in fishes support the gills and in the 

 embryos of higher animals appear only to be made over to serve a variety 

 of other functions; and so on. We shall content ourselves with but one addi- 

 tional example: the tail, clearly shown in Fig. 4.10. At this stage the 

 embryo seems prepared to provide us with as adequate a tail as that pos- 

 sessed by any lower mammal — our cat and dog companions, for exam- 

 ple. The tail reaches its maximum length when the embryo is about 6 

 weeks old; at that time its length may be about one-sixth that of the em- 

 bryo. Subsequently the tail shortens, except in rare cases (p. 76). Soon it 

 is lost to view externally, persisting as the vestigial coccyx mentioned 

 earlier (p. 43). 



At the end of the first month of life the human embryo is only about a 

 fourth of an inch long. It continues to grow and develop rapidly. By the 

 end of the second month the developing individual has distinctly human 

 appearance and arrangement of organs (Fig. 4.14). It is then no longer 

 called an embryo; it has become a human fetus. For the many fascinating 

 details of human development which could not receive attention in this 

 brief account the interested reader is referred to textbooks of human em- 

 bryology, especially to the highly readable Biography of the Unborn by 

 Margaret Shea Gilbert. 



Interferences with Recapitulation 



In the foregoing pages we have described a few examples of ways in 

 which embryos of higher animals resemble the embryos of lower animals, 

 resemblances involving surprising detail in many instances. Nevertheless, 

 a word of caution is in order about expecting too much of recapitulation. 

 In the first flush of enthusiasm following enunciation of Haeckel's version 



