ii2 GROWTH 



lies a flattened plate of cells, the embryonic disk. This disk, 

 which represents the future body, contains three layers of cells. 

 The upper layer, or ectoderm, will form the nervous system 

 and the epidermis (outer skin). The lower layer, or endoderm, 

 will form the lining of the alimentary tract. The middle layer, 

 or mesoderm, will form the remainder of the body, including 

 the muscles and connective tissues. The three germ layers are 

 therefore of fundamental importance in the development of the 

 organism. 



In the next figure (61A), the membranous roof of the amni- 

 otic cavity has been removed, showing the embryonic disk 

 viewed from above. At this stage (third week), the disk is 

 nearly one millimeter (V25 inch) in length, or smaller than an 

 ordinary pinhead. A slight groove in the primitive streak (ps) 

 is visible in the lower part of the disk. This indicates the long 

 axis of the future body. The trunk will be formed from the 

 region of this streak, while the upper (front) part of the disk 

 corresponds to the future head. 



Observation of subsequent stages, and especially experiments 

 on animal embryos (to be described later), make it possible to 

 map out on the embryonic disk what His 16 named the "organ- 

 forming germinal areas." He worked out these areas for the 

 chick. A similar (hypothetical) diagram which I ventured 23 to 

 construct for the human embryo is shown in Figure 61B. This 

 represents a somewhat later stage than 61A, since a shallow 

 neural groove (ng) indicating the brain has now appeared in the 

 midline of the head region. 



The various regions of the body, though yet undifferentiated, 

 are apparently already predetermined in the areas designated 

 on the disk.* The portions near the midline belong to the back 



* At first the various regions of the ovum and early embryo in general appear 

 in many respects undetermined, and capable of developing into different structures 

 under different conditions. The time at which the organ-forming germinal areas 

 become fixed and predetermined varies for different organs and species. The de- 

 termination is due partly to the influence of neighboring regions. For example, 

 Spemann 46 and others find that in amphibia the formation of the neural tube 



