878 



CHRONOLOGY OF HUMAN DEVELOPMENT. 



-one testicle in the inguinal canal, the eyes open, 



grammes, the descent of the testicles begins- 



the pupillary membrane often absorbed at its centre in the 28th week In the brain other 

 fissures are formed besides the primary ones. The fetus is capable of living independently. 

 At the beeinning of this month there is a centre of ossification in the os calcis. 



8th Month. -Fietus, length of body, 24-27 "8, total length 42 cm., weighing 1;5 to 2 kilos. 

 (3-3 to 4/4 ll)S -). hair of the head almn(iant > 1>3 cm - lon 8> nalls wlth a sma11 mar S m umbilicus 

 lelow the middle of the linea alba, one testicle in the scrotum. .,., 



9th Month. Foetus, length of body, 30-37, total length, 47-67 cm., weighing 2234 grammes, 

 and is not distinguishable from the child at the full period. 



Foetus at the FuU Period. Length of body, 51 cm. [20 inches], weight, 3* kilos. [7 lbs.], 

 lanugo present only on the shoulders, skin white. The nails of the fingers project beyond the 

 tips of the fingers, umbilicus slightly below the middle of the linea alba. The centre of ossifica- 

 tion in the lower epiphysis of the femur is 4 to 8 mm. broad. 



Period of Gestation or Incubation (Schenk). 



Horse, Camel, 13 months ; Rhinoceros, 18 months ; and the Elephant, 24 months. 



Limitation of the supply of to eggs, during incubation, leads to the formation of dwarf chicks. 



447. FORMATION OF THE OSSEOUS SYSTEM. Vertebral Column. The ossification of 

 the vertebra begins at the 8th to the 9th week, and first of all there is a centre in each verte- 

 bral arch, then a centre is formed in the body behind the chorda, which, however, is composed 

 of two closely apposed centres. At the 5th month the osseous matter has reached the surface, 

 the chorda within the body of the vertebra is compressed ; the three parts unite in the 1st 

 year. The atlas has one centre in the anterior arch and two in the posterior ; they unite at the 

 3rd year. The epistropheus has a centre at the 1st year. The three points of the sacral verte- 

 bra* unite or anchylose between the 2nd and the 6th year, and all the vertebrae (sacral) become 

 united to form one body between the 18th and 25th years. Each of the four coccygeal vertebrae 

 has a centre from the 1st to 10th year. The vertebra in later years produce 1 to 2 centres in 

 each process ; 1 to 2 centres in each transverse process ; 1 in the mamillary process of the 

 lumbar vertebne ; and 1 in each articular process (8 to 15 years). Of the upper and under 

 surfaces of the body of a vertebra each forms an epiphysial thin osseous plate, which may still 

 be visible at the 20th year. Groups of the cells of the chorda are still to be found within the 

 intervertebral discs. As long as the coccygeal vertebrae, the odontoid process, and the base of 

 the skull are cartilaginous, they still contain the remains of the chorda (ff. Miiller). The 

 coocygeal vertebne form the tail, and they originally project in man like a tail (fig. 665, IX, T), 

 which is ultimately covered over by the growth of the soft parts {His). 



The ribs bud out from the protovertebne, and are represented on each vertebra. The thoracic 

 ribs become cartilaginous in the 2nd month and grow forwards into the wall of the chest, 

 whereby the seven upper ones are united by a median portion (Eathke), which represents the 

 position of one-half of the sternum, and when the two halves meet in the middle line the 

 sternum is formed. When this does not occur we have the condition of the cleft sternum. 

 At the 6th month there is a centre of ossification in the manubrium, then 4 to 13 in pairs in 

 the body, and 1 in the ensiform process. Each rib has a centre of ossification in its body at 

 the 2nd month, and at the 8th to 14th one in the tubercle and another in the head. These 

 anchylose at the 14th to 25th year. Sometimes cervical ribs are present in man, and they are 

 largely develo^d in birds. 



The skull. The chorda extends forwards into the axial part of the base to the sphenoid bone. 

 The skull at first is membranous, or the primordial cranium ; at the 2nd month the basal 

 portion becomes cartilaginous, including the occipital bone, except the upper half, the anterior 

 and posterior part and wings of the sphenoid bone, the petrous part and mastoid process of the 

 temporal bone, the ethmoid with the nasal septum, and the cartilaginous part of the nose. The 

 other parts of the skull remain membranous, so that there is a cartilaginous and membranous 

 primordial crauium. 



I. The occipital bone has a centre of ossification in the basilar part at the 3rd month, and 

 one in the condyloid part and another in the fossa cerebelli, while there are two centres in the 

 membranous cerebral fossae. The four centres of the body unite during intra-uterine life. All 

 the other parts unite at the 1st to 2nd year. 



IL The poet-sphenoid. From the 3rd month it has two centres in the sella turcica, two in 

 the sulcus caroticus, two in both great wings, which also form the lamina externa of the ptery- 



