122 TEXT-BOOK OF EMBRYOLOGY. 



axis of the body (Fig. 89) . Later they are directed ventrally nearly at right 

 angles to the body axis (Fig. 92). The radial margins of the upper extremi- 

 ties are turned toward the head (cephalad) , as are the tibial margins of the 

 lower. The palmar surfaces of the hands and plantar surfaces of the feet 

 are turned inward or toward the projected sagittal plane of the body. The 

 elbow is turned slightly outward and toward the tail, the knee slightly out- 

 ward and toward the head. From these conditions it is inferred that the 

 radial side of the upper extremity is homologous with the tibial side of the 

 lower extremity, and the palmar surface of the hand with the plantar surface 

 of the foot. 



In order to acquire positions relative to the body as found in post-natal 

 life, the extremities must undergo further changes. These consist of torsion 

 around their long axes and rotation through an angle of 90 degrees. The 

 right upper extremity twists to the right, and the right lower to the left. The 

 left upper twists to the left, and the left lower to the right. At the same 

 time they swing backward through an angle of 90 degrees so that they come 

 to lie parallel with the long axis of the body. The result is that the radial 

 side of the upper extremity is turned outward or away from the sagittal plane 

 of the body and the tibial side of the lower inward or toward the sagittal plane of 

 the body. In the upper extremity this is, of course, the supine position of 

 the fore-arm in which the radius and ulna are parallel. 



Age, Length and Weight of the Body. 



The age of human embryos is a matter of importance to the embryologist 

 who desires to trace development in general or that of any organ, and to the 

 obstetrician who is interested in the scientific phase of his work. Develop- 

 ment commences when fertilization occurs and the age of the embryo should 

 be dated from that time. Unfortunately it is not possible to determine 

 exactly when fertilization takes place. This is impossible for several reasons : 

 first, because fertilization cannot be. directly observed; second, because even 

 if the time of cohabitation is known the period required by the sperm to 

 reach the ovum cannot be exactly determined; and third, because the time 

 that the ovum escapes from the ovary and passes into the oviduct is not 

 known. 



In view of these uncertain factors which cannot be controlled, the age 

 of a human embryo can be determined only within certain limits. In a few 

 cases on record the time of coitus was known, and assuming that an ovum was 

 in the tube ready for fertilization, the copulation age could be reckoned. 

 This, however, does not give the exact fertilization age, which is the actual 

 age, because it requires some time for the spermatozoa to pass through the 

 uterus and oviducts. In the Bryce-Teacher embryo, for instance, coitus 



