REPRODUCTION 12$ 



vertebrate embryos the notochord is one of the first three organs to be 

 estabUshed, the two others being the enteron and neural tube. It appears 

 simultaneously with the mesoderm. But it is not until long afterward 

 that mesenchyme, the source of cartilage and bone is produced. The 

 early formation of the notochord in the vertebrate embryo and the 

 relatively late appearance of mesenchyme, followed by cartilage and bone, 

 is in accord with the phylogenetic order. 



It is, however, not safe to assume that the embryo always adheres 

 to the phylogenetic order. Cartilage is a very ancient skeletal material. 

 The amnion is phylogenetically recent. Yet in an amniote embryo 

 the amnion develops long before any cartilage is formed. Some of the 

 departures from phylogenetic order are so glaring as to shake faith in 

 the principle of recapitulation. Yet, on the whole, it would seem that 

 the instances of adherence to the phylogenetic order are more striking and 

 more significant than the departures from it. 



It cannot be too much emphasized that the embryo of a later verte- 

 brate resembles or "recapitulates" not the adult stage of an ancestor but 

 the embryo of that ancestor. The mammal embryo does not have gill 

 pouches; it has pharyngeal pouches similar to those which, in a fish 

 embryo, develop into gill clefts. Karl Ernst von Baer in the first quarter 

 of the nineteenth century recognized the fact that the early embryos of 

 vertebrates of all classes are fundamentally similar and that the special 

 features which identify the animal as a member of a particular class 

 appear relatively late in ontogeny. 



The embryo whose development may be observed in the laboratory 

 today has had unbroken protoplasmic continuity with ancestors hundreds 

 of millions of years remote. Herein is the physical basis for recapitulation. 

 Nevertheless protoplasm seems such a fragile and labile thing that it is 

 almost incredible that it should be able to retain the impress of structures 

 which, long ages ago, lost all functional significance for the adult animal. 



