962 SPECIAL PHYSIOLOGY. 



the embryo. Within these plates, in the thoracic region, opaque lines 

 appear, which ultimately form the ribs ; where they close below in the 

 middle line, the rudiments of the separate pieces of the sternum are de- 

 veloped; these may remain ununited in the middle line, constituting a 

 rare and remarkable deformity -fissura sterni. In a similar manner, 

 the pelvic or innominate bones, the ilium, ischium, smdpubes, are found 

 near the hinder part of the body; and the scapular arch, consisting of 

 the scapula and clavicle, at the fore-part of the trunk. 



A little later, the rudiments of the limbs appear, like small knobs, 

 on each side of the trunk. In the centre of these, which consist of 

 extensions of the middle germinal layer, the rudiments of the bones 

 are soon seen, and ultimately become distinguishable as the bones of 

 the arm, forearm, and hand, or of the thigh, leg, and foot. As the 

 dorsal segments give origin, not only to the vertebrae, but also to the 

 neighboring muscles and to the covering of the cutis, so the lateral 

 plates, which produce the ribs and sternum, and the scapular and pelvic 

 arches, and also the extensions of those plates which form the future 

 limbs, not only give origin to the bones of those parts, but also to the 

 corresponding muscles and cutis. The epidermis or cuticle covering 

 all these parts, and, indeed, that of the whole body, is formed upon 

 them by the common external germinal layer. 



At first, the future skeletal parts are soft, and composed of cells 

 but slightly differentiated from the rest of the cells of the germinal 

 layer or blastoderm ; by degrees, these parts become cartilaginous or 

 membranous, and ultimately they undergo ossification. The process of 

 ossification, with its order and times of occurrence, will be hereafter 

 noticed. 



Not only are all the muscles, and also the true skin thus developed, 

 as well as the bones, ligaments, and joints, but likewise their respec- 

 tive vessels and lymphatics, and the nerves, both motor and sensory, 

 which constitute the peripheral part of the spinal and cranial nerves, 

 excepting those of special sense. The first muscles to be developed 

 are those of the vertebral grooves, next the muscles of the neck, then 

 those of the abdomen, afterwards those of the limbs, and lastly the 

 facial muscles. The limbs are at first like simple buds, derived from 

 the lateral plates ; but they soon show divisions into their respective 

 .segments, and expand and flatten at their extremities; these next ex- 

 hibit indentations corresponding with the future toes or fingers, which, 

 for a time, are webbed. The upper limb is usually developed before, 

 and more quickly than the lower one. The limbs are at first simple 

 masses of blastema, which gradually change into cartilage, bone, mus- 

 cle and skin. On the surface of the body and limbs, a layer of polyg- 

 onal epidermic cells is very early traceable; this is the commencement 

 of the cuticle. The papillae of the skin, the rudiments of the hairs, 

 feathers, nails or claws, and also of the cutaneous glands, afterwards 

 appear. 



The mammary glands, when first formed, resemble the cutaneous 

 glands, consisting of solid processes derived from the epidermic layer, 

 and penetrating the cutis; these afterwards branch out, and ultimately 

 ^become hollowed to form the mammary ducts and vesicles. 



