26 Dynamic Theory. 



a line of distinct cells running lengthwise of the bod}- and just uncle! 

 the medullaiy tube. On each side of this notochord another parallel 

 strip of cells is set off from the same skin fibrous layer. These two 

 strips afterward develop the vertebrae, and they are called the primitive 

 vertebral cords or layers. These cords begin to differentiate by break- 

 ing up into short segments which represent the future vertebrae. At 

 first they become simpty cubical shaped bodies of cartilaginous matter 

 arranged S3 T mmetrically on each side of the chorda, and developing pro- 

 gressively from front to rear. The first of these primitive vertebrae 

 formed are the first two pairs of neck vertebrae which appear almost 

 simultaneously. These are followed by the others until all are formed, 

 the number depending upon the species of animal, those of the tail 

 being the last to form. In man the total number of pairs is 33 or 34. 



In the front end of the embryo the same middle layer without detach- 

 ing the primitive vertebrae as in the rear, expands into a thin-walled ves- 

 icle which surrounds the brain and becomes afterward cartilaginous or 

 membranous in texture and at last bony. The full bone over the top of 

 the head in the human species is not completed till some months after 

 birth. 



The jaws develop from the first or anterior gill arch. The upper jaw 

 comes from a process that starts on each side of the face from the front 

 or anterior part of the gill arch. On the inner side of the gill arch a 

 cartilage is formed which develops the lower jaw. The cavity of the 

 mouth is divided into two, an upper and lower, by a horizontal partition 

 which reaches across from the two side or wing bones of the upper jaw, 

 forming the palate or roof bone of the mouth. The cavities above and 

 below the palate unite at the rear in the phaiynx. At the same time a 

 vertical plate is developed dividing the single cavity above the palate 

 into two cavities the nostrils. The intermaxillary bone is also formed 

 from the upper jaw process. It is a bone wedged in between the two 

 upper maxillary bones. It is permanent in the apes and other mam- 

 mals and is developed in the human foetus, but is afterward eliminated 

 and does not exist in the adult man. 



The limbs begin as mere buds or knobs, forming from the skin-fibrous 

 layer covered on the outside by the horn plate or skin layer. The front 

 ones appear first. They soon assume (in man and the higher verte- 

 brates) the form of a paddle with a heavy, clumsy blade and a short, 

 thick stem. Next, the front edge of the blade becomes notched or in- 

 dented with four indentations which subsequently deepen, leaving the 

 projections as digits. The digits are at first connected by a thin, web- 

 like membrane. The embryo is then web-footed, or rather finned. 

 Meantime the stem of the paddle divides into two sections, the upper 

 and lower. 



