FORM AND GROWTH 117 



unenclosed heart (h) arises in the head region. It now lies just 

 ventral to the pharynx, which is marked at this stage by three 

 gill slits, or branchial grooves. On the lateral wall, the rudi- 

 ments of the limbs (U } L) are barely visible, the upper limb 

 appearing first. 



In the next stage (62B), the embryo of 11 mm. shows a still 

 greater body curvature, apparently caused by a continued over- 

 growth of the brain and spinal cord. In an embryo of this size, 

 the volume of the brain and spinal cord is nearly three times 

 greater than the combined volume of the organs on the oppo- 

 site (ventral) side of the body axis. By the time of birth these 

 two volumes are nearly equal; while in the adult the ventral 

 organs are over three times larger than the brain and cord 

 (Table 1). In addition to this body curvature, another apparent 

 effect of the embryonic flexure in the head region, together with 

 an expansion in the facial region, is to shove downward the 

 heart and liver, which therefore appear to migrate along the 

 vertebral axis from the cervical into the thoracic region. In the 

 present stage (62B), the limbs have become larger and their 

 chief divisions visible. Even the finger rudiments are now ap- 

 pearing on the platelike hand. The large eyeball and the ex- 

 ternal ear (derived from the first branchial groove, 7) are evi- 

 dent, and a small but distinct tail (/) is visible. 



An embryo 16 mm. in length is shown in Figure 62C. This 

 approaches the end of the embryonic period, which includes the 

 first two months of gestation. By the end of this period all of 

 the more important organs are present, although their propor- 

 tions are still far from those attained later. The head is yet 

 comparatively large, though now decreasing in relative size. 

 Through increased relative growth in the facial region, and in 

 the thoracic and abdominal viscera, with a corresponding de- 

 crease in the dorsal region, the ventral flexure of the body axis 

 is being gradually straightened out. Following the embryonic 

 period comes the fetal period, which extends to the time of 

 birth. 



