THE NATURAL HISTORY OF MAN 45 



man in the serial arrangement of the vertebras, the ribs, and 

 the spinal nerves. In the lateral mesoderm a cavity develops 

 which is the beginning of the body cavity, or ccelom, and 

 later contains the heart, lungs, and viscera of the abdomen. 

 This cavity splits the mesoderm into two layers. The outer 

 layer joins with the ectoderm to form the body wall, and the 

 inner layer with the endoderm to form the wall of the ali- 

 mentary canal which in time becomes entirely enclosed by the 

 mesoderm and ectoderm of the body wall. The human 

 embryo at this stage has acquired the characteristics of a typi- 

 cal vertebrate. 



LATER PRENATAL DEVELOPMENT 



The Nervous System. The hollow neural tube early ex- 

 pands at its anterior end into three sacs (Fig. 15), the primary 

 cerebral vesicles, or as they are often called the fore-, mid-, 

 and hind-brain, and later the first and third sacs partially sub- 

 divide making altogether five enlargements. From the walls 

 of these five sacs and from the remainder of the neural tube 

 are developed all parts of the brain and spinal cord and nearly 

 all parts of the cranial and spinal nerves which run to the 

 peripheral regions of the head and body. The developing 

 brain increases in length rapidly and bends in three places, the 

 cervical flexure remaining permanently. A small sac grows 

 out on each side of the hollow anterior brain vesicle and ulti- 

 mately forms the retina of the eye (Fig. 15), which is thus a 

 part of the brain projected to the surface of the head so the 

 light can reach it. By thickenings, thinnings, and outgrowths, 

 in different parts of the walls of the cerebral sacs, and by the 

 development of myriads of nerve cells and fibers growing in 

 many directions, the various parts of the brain are ultimately 

 formed. Another sac grows out from each side of the fore- 

 brain in front of the developing retina which (Fig. 15), in 

 man especially, possesses the power of extensive growth. This 

 sac expands in all directions but especially backward, spreading 



