DEVELOPMENT OF THE EXTERNAL FORM OF THE BODY. 151 



tween the anlagen are marked by radial depressions on the flat surfaces; the 

 anlagen themselves are the elevations between the depressions (Figs. 125, 126). 

 The anlagen grow rapidly in thickness and length, thus producing not only an 

 apparent deepening of the radial depressions but also indentations around the 

 distal free borders of the limb buds (Fig. 126). The depressed areas produce a 

 web-like structure between the digits, resembling the web in some aquatic 

 animals. The web does not keep pace with the digits, however, and is soon 

 confined to the proximal ends of the latter. In length the fingers grow slightly 

 more rapidly than the toes and thus become somewhat longer. From the 

 seventh week on, the thumb and great toe become more and more widely sepa- 

 rated from the index finger and the second toe respectively (Figs. 128, 130, 131). 



As the limb buds become elongated during the earlier stages of development, 

 they assume a position with their long axes nearly parallel with the long axis of 

 the body, and are directed caudally (Fig. 125). In later stages they are directed 

 ventrally and their long axes are nearly at right angles to the long axis of the 

 body (Fig. 126). The radial margins of the upper extremities are turned 

 toward the head, as are the tibial margins of the lower. The palmar surfaces 

 of the hands and the plantar surfaces of the feet are turned inward or toward 

 the body. The elbow is turned slightly outward and toward the tail, the knee 

 slightly outward and toward the head. From these conditions it may be con- 

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

 side of the lower; that the palmar surface of the hand is homologous with the 

 plantar surface of the foot; and that the elbow is homologous with the knee. 



In order to acquire the position relative to the body as found in postnatal 

 life, the extremities must undergo further changes. These consist essentially 

 of tortions around their long axes. The right upper extremity turns to the 

 right, the right lower turns to the left. The left upper extremity turns to the 

 left, the left lower turns to the right. At the same time the extremities rotate 

 through an angle of ninety degrees and again come to lie parallel with the long 

 axis of the body. The result is that the radial sides of the upper extremities are 

 turned outward (away from the sagittal plane of the body) and the tibial sides 

 of the lower are turned inward (toward the sagittal plane of the body) . In the 

 upper extremity this is, of course, the supine position in which the radius and 

 ulna are parallel. 



Age and Length of Embryos. 



AGE. Certain general conclusions regarding the age of embryos have been 

 formulated by His (Anatomic menschlicher Embryonen, 1882) and accepted 

 for the most part by embryologists. These as stated by His are as follows: 



i. Development begins at the time of impregnation, that is, at the moment 

 when the male sexual element enters the ovum and fertilizes it. 



