298 Basic Structure of Vertebrates 



the gastrula stage in ontogeny, for some such form as the gastrula is 

 the necessary precursor of any adult metazoan which has a skin (ecto- 

 derm) and a digestive tube (endoderm). 



"Determination" in Germ-Layers. After the germ-layers have 

 been established, the development of organs proceeds with a high 

 degree of uniformity in vertebrates of all classes. It would appear 

 that each germ-layer is rigidly "determined" — capable of producing 

 certain organs and no others. From fish to mammal, brains and the 

 deep-seated autonomic ganglions are ectodermal; livers and lungs are 

 endodermal. Indeed, there is so much evidence of rigid necessity that 

 the germ-layer origin of organs is generally regarded as the safest 

 criterion in judging their phylogenetic relationships. But at early 

 stages of development the embryonic material is not so rigidly "deter- 

 mined." If one cell of the two-cell cleavage stage of Amphioxus or an 

 amphibian is removed, the remaining cell may produce a complete 

 embryo, nearly or quite normal except in size. A large portion of the 

 early blastoderm of a fish egg may be destroyed and yet the surviving 

 fraction will develop into a normal young fish. By appropriate opera- 

 tions at early stages of embryos, both invertebrate and vertebrate, 

 it has been proved that a certain region of germ material may produce 

 structures other than those which it normally produces. 



"Recapitulation." Ernst Haeckel in 1874 asserted that "On- 

 togeny is a brief repetition of Phylogeny." The embryology of the 

 succeeding three or four decades was inclined to overwork the idea, 

 expecting — and often claiming — to find "repetition" in minute details 

 of ontogeny. Thus the theory was brought into some disrepute. 



There are many facts which seem to justify, or even compel, the 

 conclusion that there is a direct relation between the phylogenetic and 

 the ontogenetic order. The notochord is unquestionably the most 

 ancient axial skeleton. It develops in the early embryo of all modern 

 vertebrates. In sharks cartilaginous vertebrae are developed around 

 the notochord. In reptiles, birds, and mammals, the notochord be- 

 comes surrounded by cartilaginous vertebral structures which are later 

 replaced by bone. Nowhere else in the body is bone preceded by any 

 such structure or tissue as the notochord. Many bones (dermal) de- 

 velop directly from mesenchyme. In general, formation of bone is not 

 necessarily dependent on preexisting cartilage or notochord. Sharks 

 have an all-cartilaginous skull. In reptile, bird, and mammal, the adult 

 skull is nearly all bone. But the embryo develops a cartilaginous skull 

 similar to that of a shark. Later the cartilage is almost wholly replaced 

 by bone and numerous dermal bones are developed superficially to the 

 cartilage. This succession of materials or structures — notochord, carti- 

 lage, bone — in higher vertebrates has been generally regarded as an 



