THE EIGHTH AND NINTH MONTHS. 509 



The skin is paler than before. The subcutaneous connective 

 tissue is filled with fat, giving roundness and firmness to the 

 body and limbs. The hair is thick, long, and fairly abundant 

 on the head, while the down has begun to disappear from the 

 body. 



The umbilicus is almost exactly in the middle of the body, 

 or slightly behind this point. Both testes are, as a rule, in the 

 scrotum, which has now a corrugated surface. 



Ossification has commenced in the centre of the cartilage at 

 the lower end of the femur. This is the first epiphysial ossifica- 

 tion to appear in the body, and is often the only one present at full 

 time. Ossification has sometimes commenced in the proximal 

 epiphyses of the tibia and humerus ; but while the presence of 

 these centres is a sure sign of full time having been reached, 

 their absence does not, without further evidence, indicate pre- 

 mature delivery. 



DEVELOPMENT OF THE NERVOUS SYSTEM. 



The general history of development of the human nervous 

 system is the same as in other Vertebrates. Certain points, 

 especially in connection with the brain, will require detailed 

 notice ; and, with regard to the histological development of the 

 nervous elements, recent researches by His, and others, have 

 shown that human embryos are well suited for the most minute 

 investigations. 



1. The Brain. 



a. General account. It will be convenient to give first a 

 general account of the development of the brain, and of its con- 

 dition at successive stages, and then a more detailed descrip- 

 tion of parts, such as the cerebral hemispheres, which are of 

 special interest. 



The second week. In the youngest human embryos, such as 

 His' embryos E and SR (Figs. 176, 178, and 179), estimated as 

 about thirteen days old, the neural groove is widely open along 

 its whole length, but by comparison with later embryos it is 

 possible to determine, even at this stage, the several regions of 

 the brain. 



Thus, in Fig. 179, the dorsal concavity, opposite the refer- 



