ii 4 GROWTH 



or dorsal region of the body, while the outlying zone will form 

 the future lateral and ventral body walls. The method by which 

 the flattened embryonic disk is transformed into the body was 

 shown by His, who was the pioneer student of developmental 

 mechanics. Through a process of unequal growth in the differ- 

 ent regions and constituent layers, the disk becomes gradually 

 folded into a tube, with the upper layer of the disk forming the 

 outer skin (epidermis), and the lower layer becoming the lining 

 of the alimentary canal. The head and tail ends are also folded 

 ventralward, so as to close up the ends of the tube. 



/. Law of Developmental Direction 



If we compare the chief regions of the body already located 

 on the embryonic disk, it will be noted that their areas are far 

 from proportional to the sizes of the corresponding parts which 

 they are later to form. In the first place, the head region is rela- 

 tively enormous, occupying nearly half the total area. The neck 

 or cervical region is about half as large as the head, and the 

 other segments of the trunk decrease progressively in size 

 toward the tail region. In general, the embryonic head is differ- 

 entiated first j and from the primitive streak region the neck, 

 thorax, abdomen, and pelvis are budded off successively. In the 

 second place, the area near the midline (future dorsal or back 

 region) is relatively large; while the peripheral zone (ventro- 

 lateral body wall) is at first very small. 



Growth correspondingly appears at first relatively more rapid 

 in the dorsal head region, and less rapid in the parts located 

 ventralward and tailward. Only gradually, through progressive 

 changes in the relative growth rate, do the various regions of 

 the body later attain their adult proportions. These principles 

 may be designated as the "law of developmental direction" 



from the neural groove region of the ectoderm is dependent upon the presence 

 of the underlying ecto-mesoderm. If transplanted previous to gastrulation, the 

 ectoderm which normally would form the neural tube may be caused to differ- 

 entiate instead into other structures. Thus ectoderm may even produce somites, 

 pronephros, or intestine, which normally come only from mesoderm or endoderm. 



