EVOLUTION AS SEEN IN EMBRYONIC DEVELOPMENT 69 



functions as a kidney), then a mesonephros (which functions as a kidney 

 for a time in the embryo), and finally a metanephros. Why is all this past 

 history repeated? Why not develop the metanephros at once and have 

 done with it? Probably we see here evidence of a chain reaction of the 

 type mentioned above. It may well be that each kidney serves in turn as 

 an organizer, inducing formation of structures which follow it in time of 

 development. 



In conclusion, we see that ancestral embryonic structures are retained 

 when their retention serves some useful function in promoting embryonic 

 development. This function is frequently that of organizers, making pos- 

 sible the forging of successive links in the chain reaction which consti- 

 tutes embryonic development. 



Another confusing aspect of embryonic development is the fact that the 

 time sequence of stages is frequently modified or even reversed. The 

 mammalian placenta, for example, is a relatively recent development in 

 evolutionary history — the invertebrate, fish, amphibian, and reptilian 

 ancestors of mammals all lacked it. Not until the mammalian status was 

 reached, necessitating nourishment of the embryo within the uterus of the 

 mother, did a placenta (Fig. 4.10) develop. We might expect, therefore, 

 that the placenta would develop late in the embryonic life of mammals. 

 But if that were the case how would the mammalian embryo obtain food 

 during its earlier development? Since the mammalian ovum has no yolk, 

 connection with the mother's blood stream must be made almost at once. 

 Hence, as already noted, the placenta is one of the first features of a 

 mammalian embryo to appear, despite its arising late in evolutionary his- 

 tory. This phenomenon illustrates the fact that an embryo must at all times 

 be adapted to its embryonic environment and to the requirements im- 

 posed upon it of securing the necessities of life. Attainment of such adapta- 

 tion has frequently destroyed recapitulation of ancestral embryonic stages 

 or distorted the sequence in which the stages occur. In some cases the 

 differing adaptations of embryos or larvae of two related forms may re- 

 sult in these young stages being more unlike than the adults are. This gives 

 rise to exceptions to von Baer's principle (p. 50). 



Recapitulation and the Origin of the Metazoa 



In the examples of recapitulation by the human embryo we have seen 

 that the repetition of stages confirms the idea that remote ancestors of man 

 (and other mammals) were fishes, and that subsequently reptiles occupied 

 a place in the lineage leading from fishes to mammals. It is noteworthy 



