CHAPTER XXXII 



DEVELOPMENT OF THE OVUM 



Development of the cavities and layers of the trunk in the chick Development of the skeleton, 

 muscular system and skin Notochord Vertebral column etc. Development of the 

 nervous system Development of the digestive system Development of the respiratory 

 system Development of the face Development of the teeth Development of the 

 genito-urinary apparatus Development of the urinary system Development of the 

 external organs of generation Development of the circulatory system The first, or vitelline 

 circulation The second, or placental circulation Development of the heart Peculiari- 

 ties of the foetal circulation The third, or adult circulation. 



THE product of generation retains the name of ovum until the form 

 of the body begins to be apparent, when it is called the embryo. At 

 the fourth month, about the time of quickening, it is called the foetus, 

 a name retained during the rest of intra-uterine life. The membranes 

 are appendages formed for the purposes of protection and nutrition ; 

 and the embryo itself, in the mammalia, is developed from a restricted 

 portion of the layers of cells resulting from the segmentation of the 

 ovum. 



The formation of the blastodermic cells and the appearance of the 

 groove which is subsequently developed into the neural canal have 

 already been described. At this portion of the ovum, there is a 

 thickening of the blastoderm, which then presents three layers, the 

 mesoblast the thickest and most important being developed between 

 the epiblast and the hypoblast. The earliest stages of development have 

 been studied most successfully in the chick ; and it is probable that the 

 appearances here observed nearly represent the earlier processes of 

 development in the human subject. 



DEVELOPMENT OF THE CAVITIES AND LAYERS OF THE TRUNK IN THE 



CHICK 



As an introduction to a description of the development of special 

 organs in the human subject and in mammals, it will be found useful to 

 study the first stages of development in the chick, which will give an 

 idea of the arrangement of the different blastodermic layers and the way 

 in which they are developed into the different parts of the trunk, with 

 the mode of formation of the great cavities. The figures by which this 



816 



