Reproduction 299 



instance of "recapitulation." Some evidence from paleontology, how- 

 ever, indicates that bone may be more ancient than cartilage and 

 suggests that the cartilaginous skeleton of sharks may have resulted 

 from "degeneration" of an originally bony skeleton. 



The pharyngeal region of the mammalian embryo exhibits an ap- 

 parently good case of "repetition." With its paired visceral pouches, 

 cartilaginous visceral arches, and numerous aortic arches, it is essen- 

 tially fishlike. During later development the region undergoes a com- 

 plex metamorphosis, in the course of which some transitory reptilian 

 features appear, especially in the aortic arches. Here, as in numerous 

 other instances (e.g., the succession of kidneys — pro-, meso-, meta- 

 nephros), the embryo pursues a course of development which is in- 

 direct, complex, and laborious, involving breaking down structures 

 already formed and replacing by others, to arrive finally at a condition 

 simpler than that at the start. Such facts are difficult to explain other- 

 wise than as due to some inherited necessity. 



It should be emphasized that the embryo repeats, not the adult 

 stage, but an embryonic stage of an ancestor. The mammalian embryo 

 does not have gill-pouches; it has pharyngeal pouches similar to those 

 which, in a fish, develop into gill-clefts. 



Accepting the basic idea of "recapitulation," it must be admitted 

 that the ontogenetic record is incomplete and exhibits many departures 

 from the phylogenetic order. Cartilage is an ancient skeletal material; 

 the amnion is comparatively recent. Yet in a modern embryo the 

 amnion develops long before any cartilage is formed. The record is 

 further confused by the insertion of new ("cenogenetic") structures 

 into early stages of an embryo. The amnion and allantois are ceno- 

 genetic structures. The notochord of a mammalian embryo is relatively 

 ancient or "palingenetic." 



The embryo whose development is studied in our laboratory today 

 has had unbroken protoplasmic continuity with ancestors hundreds of 

 millions of years ago. Herein is a material basis for "recapitulation." 

 Nevertheless, it is almost incredible that a substance chemically so 

 active and structurally so delicately complex as protoplasm should so 

 tenaciously retain the impress of structures which, long ages ago, lost 

 all functional significance for the adult animal. Protoplasm, or the 

 organism constituted of it, is a paradox. It exhibits extraordinary 

 capacity for adaptation to new situations and modes of living, along 

 with inability to discard obsolete embryonic processes and structures 

 whose retention more or less complicates and hampers embryonic de- 

 velopment. Vestiges of obsolete organs may even persist on into the 

 adult stage, sometimes (e.g., the human vermiform appendix) proving 

 to be even worse than useless. 



