FETAL MOVEMENTS. DEVELOPMENT OF THE OSSEOUS SYSTEM. 983 



given: Colibri, twelve days; hen, duck, twenty-one days; goose, twenty-nine 

 days; stork, forty-two days; cassowary, sixty-five days; mouse, three weeks; 

 rabbit, hare, four weeks; rat, five weeks; hedgehog, seven weeks; cat, marten, 

 eight weeks; dog, fox, polecat, nine weeks; badger, wolf, ten weeks; lion, fourteen 

 weeks; pig, seventeen weeks; sheep, twenty-one weeks; goat, twenty-two weeks; 

 roe, twenty-four weeks; bear, small apes, thirty weeks; deer, from thirty-six 

 to forty weeks; man, forty weeks; horse, camel, thirteen months; rhinoceros, 

 eighteen months; elephant, twenty-one months. Limitation of the supply of 

 oxygen to the incubating egg of birds is followed by dwarf -formation. 



Various intrauterine movements of the fetus are discernible through the 

 abdominal wall of the mother: extension-movements of the trunk, movements ot 

 the extremities, and in the later period of pregnancy (and during labor) a regular 

 rhythmical movement of the respiratory muscles, recurring at intervals and 

 usually continuing for some time. In addition the fetus makes sucking and 

 swallowing movements. 



DEVELOPMENT OF THE OSSEOUS SYSTEM. 



Spinal Column. The ossification of the vertebrae begins between the eighth 

 and the ninth week, a center appearing first in each half- arch, then a center, 

 probably consisting of two lying close together, in the body behind the chorda. In 

 the fifth month the bony tissue has reached the surfaces, the chorda in the body 

 is displaced. The three pieces unite in the first year. The atlas has a center in 

 the anterior arch and two in the posterior arch; union occurs in the third year. 

 A center appears in the epistropheus (axis) during the first year. The three 

 points of the sacral vertebrae unite between the second and the sixth year; and 

 all the vertebrae (sacral) join together between the eighteenth and the twenty- 

 fifth year. The four coccygeal vertebrae receive each a center of ossification 

 between the first and the tenth year. The vertebrae produce further in later life one 

 or two centers in each spinous process, one or two in each transverse process, one 

 in the mammillary process of the lumbar vertebrae, and one center in some of 

 the articular processes between the eighth and the fifteenth year. Each surface 

 of a vertebra develops further an epiphysis-like, thin plate of bone, which may 

 still be visible at the age of twenty years. Groups of chorda-cells persist in the 

 adult in the intervertebral discs. So long as the coccygeal vertebrae, the odontoid 

 process of the axis and the base of the skull are cartilaginous, they contain remains 

 of chorda. The coccygeal vertebrae form the tail, as the continuation of which 

 an invertebrate caudal filament is prolonged. The coccyx consists originally 

 in man of a free projecting tail, vertebral tail (Fig. 376, IX, T), which later becomes 

 covered and enclosed by the growth of the soft parts over it. Rarely, a free, pro- 

 jecting tail persists; if the caudal filament alone remains free, there is formed the 

 so-called soft tail. 



The number of rudimentary vertebrae is at first small, then larger than even 

 in adults ; and, finally, again smaller. Eventually the embryo has 2 5 true vertebrae, 

 the ilium fusing with the twenty-sixth. Later, the ilium moves so far forward 

 that the twenty-fifth vertebra becomes the first sacral. The persistence of 25 

 true vertebrae is to be regarded as a developmental defect. 



The ribs bud out from the primitive vertebrae; their first rudiments reside 

 in each vertebra. The thoracic ribs become catilaginous in the second month 

 and grow forward into the chest- wall, the upper seven being joined together by 

 a median strip of cartilage. The latter represents the rudimentary half of the 

 sternum; by the union of the rudiments of the two sides in the median line the 

 sternum is formed. The developmental defect of fissure of the sternum occurs 

 in some howling apes in which the manubrium is permanently divided. The 

 lower, false ribs normally exhibit, to a certain extent, a fissure of the sternum; 

 openings in the sternum as the remains of a fissure are frequent. 



In the sixth month a center of ossification appears in the manubrium; then 

 from 4 to 13, in pairs, in the gladiolus, and one in the ensiform process. Each 

 rib acquires a center of ossification in the body in the second month; between 

 the eighth and the fourteenth year one each in the tubercle and the head of the 

 bone; fusion takes place between the fourteenth and the twenty-fifth year. The 

 rudimentary ribs in front of the transverse processes in the neck become the 

 anterior portions of these processes. Rarely isolated, short, true cervical ribs 

 persist in conjunction with the sixth and seventh cervical vertebrae (in birds the 

 cervical ribs are better developed). In the lumbar region the cartilaginous 



