146 TEXT-BOOK OF EMBRYOLOGY. 



ing to Jordan one of these is haemogenic and the other osteolytic. The 

 former originates from enlarged hasmoblasts and may be regarded as repre- 

 senting centers of intense haemopoiesis, giving rise to erythrocytes. The 

 osteolytic giant-cells (osteoclasts) arise more frequently from fused portions 

 of the marrow reticulum, less often from fused osteoblasts, and are always 

 multinucleated. Arey maintains in his more recent work that the so-called 

 osteoclasts usually arise by fusion of old and basophilic osteoblasts, the 

 cytoplasm of the syncytial mass becoming acidophilic. Arey also holds 

 that this type of giant-cell is not a specific agent in bone resorption. 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. 136), 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 fcetal life, leaving but a slight trace of 

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



* Ectoderm 

 Mesoderm 



Entoderm 



FIG. I2i. 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. 121) 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 chorda! sheath. On account of its 

 position the notochord naturally becomes embedded in the developing vertebral 



