CHAPTER XY1. 



DEVELOPMENT OF THE EMBRYON THE OSSEOUS, MUSCULAR, 



CUTANEOUS, AND NERVOUS SYSTEMS. 



General view of the development of the embryon Development of the cavities 

 and layers of the trunk in the chick External blastodermic membrane 

 Intermediate membrane, in two layers Internal blastodermic membrane 

 Neural canal Chorda dorsalis Primitive aortaB Vertebrae Origin of the 

 Wolffian bodies Pleuro-peritoneal cavity Development of the skeleton 

 Ossification of the skeleton Development of the muscles Development of 

 the skin Development of the nervous system Development of the enceph- 

 alon Development of the organs of special sense Cartilage of Meckel. 



THE product of generation retains the name of' ovum un- 

 til the form of the body begins to be apparent, when it is 

 called the embryon. At the fourth month, about the time 

 of quickening, it is called the foetus, a name which it retains 

 during the rest of intra-uterine life. The membranes which 

 we have described in the preceding chapter are appendages 

 developed for the purposes of protection and nutrition ; and 

 the embryon itself, in the mammalia, is developed from a 

 restricted portion of the layers of cells resulting from the 

 segmentation of the vitellus. 



In the preceding chapter, we have described the forma- 

 tion of the blastodermic cells and the appearance of the 

 groove which is subsequently developed into the neural ca- 

 nal. At this portion of the ovum, there is a thickening of 

 the blastoderm, which then presents three layers, the middle 

 layer, the thickest and most important, being developed from 

 the opposite surfaces of the external and the internal layer. 1 

 We have to study, then, the changes which take place in 



1 See page 360. 



