182 TEXT-BOOK OF EMBRYOLOGY. 



There is also just as much uncertainty in regard to the origin of red blood cells 

 in the marrow. At an early stage nucleated red cells are present, and from 

 this time on, the marrow affords a place at least for their proliferation, for in 

 the adult marrow all the nucleated forms are found, as well as the non-nucleated. 

 The origin of the marrow cells myelocytes is not known. The fibrous 

 part of the osteogenetic tissue assumes a reticular structure and forms the 

 reticulum of the marrow. 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. 172), so that these form the greater part of the marrow. Such a process oc- 

 curs 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 



" ~ - ~ Ectoderm 

 --- - Mesoderm 



" ~~ 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- J 59) and tnen 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 



