Appendix A. 
177 
Vra. ORGANOGENESIS 
The basic structures and relations of all the major organ systems of 
the body emerge during the fourth through the eighth weeks of 
embryonic development. First, the embryo folds in several ways so 
that the flat linear structure distinguished by neural tube flanked by 
somites become roughly C-shaped. The effect of this is to bring the 
regions of the brain, gut, and other internal organs into their famiUar 
anatomical relations. During the fourth week the neural pores, the 
ends of the neural tube “zipper,” close. First the one at the cranial or 
head end, which is called the anterior or rostral pore, closes, and later 
the caudal or tail-ward pore closes. Closure of the neural pores 
completes the closure of what will become the central nervous 
system. Also during the fourth week, limb buds become visible, first 
buds for arms and later for legs. Further, two accumulations of cells 
along the neural tube become distinguishable: the alar plate and the 
basal plate. Cells of the alar plate go on to become mostly sensory 
neurons, while basal plate cells give rise mostly to motor neurons. 
Already while the neural tube is closing, its walls along the cranial 
area are thickening to form early brain structure. Cranial nerves, for 
example the nerves for the eye and for the muscles of the face and 
jaw, also are beginning to develop at this time. The embryonic brain 
develops rapidly in both size and structure especially during the fifth 
week, and the optic cup that will form the retina of the eye becomes 
visible as well. 
DC. CONCLUSION AND CONTINUATION 
Embryonic development continues with the emergence and 
differentiation of organs, the skeleton, hmbs, and digits, and with the 
development of the face and further differentiation and integration 
throughout the body. The development discussed above is 
summarized briefly in Table 1.® But development continues, and is a 
continuous process, past the eight-week mark, when the organism is 
no longer called an embryo and instead is called a fetus. Although 
the basic elements of the body plan have been established during 
embryogenesis, a great deal of development of that body plan, 
refinement and integration, continues in the fetal stage, also called 
phenogenesis (emergence of the normal appearance of the body). 
Development continues after birth as well. 
PRE-PUBLICATION VERSION 
