22 



MORPHOGENESIS 



As the muscles become differentiated, nerves grow to them from a definite 

 number of spinal segments (fig. 25). 



At first each limb plate is so placed that one of its surfaces looks dorsally and the other ven- 

 trally, and one border (that corresponding to the thumb or great toe) is anterior (i. e., cranial) 

 and the other posterior (caudal). Later, however, each limb becomes bent caudaUy through 

 about ninety degrees, so that the limbs whose long axes were at first at right angles to the long 

 axis of the body come to lie parallel to that axis. In addition there occurs a rotation of each 

 fore-Mmb in such a manner that the thumb turns latero-dorsally, while in the lower limb the 

 direction of the movement is exactly the opposite, the great toe turning ventro-medially. As 

 a result there is an apparent reversal of the surfaces in the two lirnbs, the flexor muscles of the 

 arm reaching on the surface which is directed anteriorly, while in the lower limb the corre- 

 sponding muscles occupy the posterior surface. The dorsum of the foot and the great toe side 

 correspond respectively to the back and thumb side of the hand, the tibia corresponds to the 

 radius and the fibula to the ulna. The limb anlage soon becomes divided into three primary 

 segments. The distal segment (hand or foot) is a flattened rounded disc, in which the digits 

 soon appear (fig. 26). The proximal portion forms the forearm or leg and the arm or thigh. 

 In general, the extremities follow the law of cranio-caudal and dorso-ventral (proximo-distal) 

 development. 



Fig. 28A. — Face op Human Embryo^of 

 ABOUT 8 MM. (His.) 



Nasal fossa 

 Lateral nasal process 

 jlobular process 

 Maxillary process 

 Mandibular process 



Fig. 28B. — Face of Human Embryo 

 AT Stage Slightly Later than 28A. 

 (After Kallius.) 



Nasal fossa 



Lateral nasal 



process 

 Globular process 

 Maxillary process 



Mandibular 

 process 



Prenatal Growth in Length and Weight 



Prenatal growth. — The prenatal growth of the human body in length and weight is indi- 

 cated in the j)receding table. According to Hasse, the age of the fajtus may be estimated from 

 its total length as follows. Before the fifth month, the square of the age in (lunar) months gives 

 the length in centimetres. After this, the age in months multiplied by five gives the length. 

 This gives approximate results, except for the first month. 



While the growth in absolute weiglit increases from month to month, it is important to note 

 that the real (relative; growth rate rapidly diminishes. The ovum increases in weight^during 

 the first month about 1000 times, or 100,000 per cent, (not including the extra-embryonic 

 structures). This rate diminishes rapidly, however, so that the increase during the last foetal 

 month is only about 33 per cent. 



The continuation of growth in length and weight during the postnatal period is shown in 

 the following cliart (fig. 30). 



The following chart is ba,sed upon data from Camerer (1-5 yrs.), Porter (6-17 yrs.), and 

 Roberts (18-20 yrs.), showing the average postnatal growth in height and weight by sexes. 

 The average height at birth is about .^O cm. (20 inches); weight, about 3200 g. (7 pounds). The 

 male is slightly heavier and taller than the female, except during the acceleration at the period 



♦ 270 days (MaU). 



