178 



TEXT-BOOK OF EMBRYOLOGY. 



blood cells, mononuclear and with slightly basophilic cytoplasm. They 

 probably arise from mesenchymal cells or even from the elements of the 

 reticular tissue, and probably also constitute the cells from which not only 

 the myelocytes but also red and white blood cells are derived. In forming 

 the myelocyte series they acquire certain granules in the cytoplasm which 

 may be neutrophile, acidophile or basophile, giving the cell its distinguish- 

 ing character. The bone marrow in the adult is normally the only source 

 of erythrocytes and one of the sources of leucocytes. Further discussion 

 of these cells will be found on page 271. In young marrow there is little 

 or no fat present, but in later life many of the connective tissue cells are 

 transformed into fat cells (p.i68),so that these form the greater part of 

 the marrow. Such a process occurs most extensively in the shaft of the long 

 bones and gives rise to " yellow" marrow. In the heads of the long bones, 

 in the ribs, and in the short bones the marrow retains its earlier character 

 and is known as "red" marrow. 



THE DEVELOPMENT OF THE SKELETAL SYSTEM. 

 The Axial Skeleton. 



The Notochord. The notochord (chorda dorsalis) constitutes the 

 primitive axial skeleton of all Vertebrates, yet it differs from the other skeletal 

 elements in that it is a derivative of the entoderm. In man it is merely a tran- 

 sient structure and disappears early in foetal life, leaving but a slight trace of 

 itself in the intervertebral disks. In embryos of 2-3 mm. the cells of the 



Mesoderm"" 



Anlage of 

 notochord 



Entoderm 



FIG. 159. From transverse section of human embryo with 8 pairs of 

 primitive segments (2.69 mm.). Kollmann. 



entoderm just ventral to the neural groove become slightly differentiated 

 (Fig. 159) and then form a groove with a ventral concavity. The groove closes 

 in, becomes constricted from the parent tissue (entoderm) and lies just ventral to 

 the neural tube, where it soon becomes surrounded by mesodermal tissue. This 

 structure is the notochord and constitutes a solid, cylindrical cord of cells 

 extending from a point just caudal to the hypophysis to the caudal extremity of 

 the embryonic body. In embryos of 17-20 mm. the mesodermal tissue around 

 the notochord becomes modified to form the chordal sheath. On account of its 

 position the notochord naturally becomes embedded in the developing vertebral 



