THE DEVELOPMENT OF THE INDIVIDUAL 533 



originate as membranes close to the epidermis. These membranes grad- 

 ually sink deeper into the head region and ossify directly. These form 

 the bones of the top of the head, the face, and the jaws. Cartilage 

 bones, which make up the rest of the bones of the body, are laid down 

 first as cartilages. These have very nearly the shape of the future 

 bones. As development proceeds, calcium salts are deposited in ossi- 

 fication centers and the cartilage is replaced by bone. In the long bones 

 the ossification centers are in the middle of the shaft and in the heads 

 at either end. As long as these three centers of bone formation are 

 separated by a layer of cartilage, the bones can continue to grow in 

 length. Sometime between the ages of sixteen and twenty-one years 

 of age most of these centers fuse and growth in height can no longer 

 take place. X-rays can show the exact time when growth stops because 

 these cartilage areas show plainly on x-ray films. At the time of birth 

 the bones of the skull are only partially ossified and the head of the 

 infant is bent to conform to the narrow birth canal in the pelvis of its 

 mother. A baby is born with soft spots in its skull where the mem- 

 branes have not finished ossification. 



The Muscular System. Muscles make up about 40 per cent of a 

 person's body by weight, which is more than any other single system. 

 Practically all of the muscles are derived from the mesoderm (the 

 muscles of the pupil of the eye and the muscles which cause the hair to 

 stand on end are ectodermal). As we learned in Chapter 22, there are 

 three kinds of muscles and each is formed somewhat differently. 



Striated or skeletal muscle arises chiefly from the somites that are 

 such a conspicuous feature of the early embryo. The main part of each 

 somite is the myotome which gradually enlarges to form the muscles 

 of the body wall, both the back muscles and the ventral muscles. In 

 the frog the original myotomes can be observed in many of the muscles 

 especially in the rectus abdominis of the abdomen. Greek sculptors, 

 with their remarkable abilities to depict the human form, carved athletes 

 with the same lines across the abdominal region, a feature which is 

 rarely visible externally in human beings. The skeletal muscles of the 

 arms and legs of the frog are formed from the same mass of mes- 

 enchymal cells which forms the bones. 



Smooth muscles of the digestive tract are formed from the wander- 

 ing mesenchymal tissue which migrates between the endoderm of the 

 gut and the splanchnic mesoderm of the coelom. The cells gradually 

 elongate and form the spindle-shaped cells that bring about peristaltic 

 movements. Similar smooth muscles form around the blood vessels, 

 the excretory ducts, and the urogenital ducts. 



