NOTOCHORD 83 



In an embryo measuring 5.5 mm. the notochord is a rod of cells surrounded by a 

 thin notochordal sheath. A cross section contains about eight wedge-shaped cells. 

 In an embryo measuring 9 mm. it is larger, and a cross section shows about fifteen 

 cells at the periphery, and three or four at the center. In an embryo of n mm. the 

 cells have lost all definite arrangement and are more or less vacuolated. The vacuoles 

 increase in size and number, and are found to contain mucin or a gelatinous mucin- 

 like substance. In an embryo measuring 17 mm. the cell walls, which up to this 

 time have remained intact, are breaking down (or being absorbed) and the mucin 

 escapes from the vacuoles. The cells are united by strands of cytoplasm and the 

 notochordal tissue now resembles mesenchyma. The syncytial network continues 

 to enlarge, both by growth, and by the formation of a greater number of vacuoles. 

 In a much older embryo (250 mm.) the formerly continuous peripheral sheet of 

 syncytial tissue is broken in many places by large masses of mucin. In the center 

 of this accumulation, the slender syncytial network seems suspended (cf. Fig. 70). 

 In the adult the syncytium has become divided into groups of vacuolated cells 

 imbedded in a gelatinous matrix. Thus it acquires a resemblance to cartilage in 

 several particulars, but it should be regarded as a distinct tissue. 



The human notochord undergoes a development similar to that of 

 the pig. After it has ceased to be an epithelioid rod of cells, its most 

 characteristic condition is that shown in Fig. 70, which includes a portion 

 of the nucleus pulposus from an embryo of the fifth month. The noto- 

 chordal tissue forms a vacuolated syncytium suspended in the gelatinous 

 matrix, which, at the periphery of the nucleus pulposus, is bounded by a 

 structureless membrane. Very rarely the notochord is the source of 

 tumors which are composed of tissue similar to that normally found 

 within the nucleus pulposus. 



BONE. 



Bone develops relatively late in embryonic life, after the muscles, 

 nerves, vessels, and many of the organs have been formed. The skeleton 

 at that time consists of hyaline cartilages, which are later replaced by 

 the corresponding bones of the adult. According to KSlliker, Robert 

 Nesbitt was the first to point out that the bones are not indurated or 

 transmuted cartilages, but are new formations, produced around the 

 cartilages which are later destroyed. Moreover, in his ' ' Human Osteogeny 

 Explained in Two Lectures" (London, 1736), Nesbitt showed that 

 certain bones develop directly from connective tissue without having 

 been preformed in cartilage. These are now called membrane bones in 

 distinction from cartilage bones. The membrane bones are the bones 

 of the face and the flat bones of the skull. They include the interparietal 

 or upper part of the occipital, the squamous and tympanic parts of the 

 temporal, the medial pterygoid plate of the sphenoid, the parietal, frontal, 

 nasal, lachrymal, zygomatic (malar) and palate bones, together with the 

 vomer, maxilla and almost the entire mandible. Nesbitt correctly 



