56 



THE EARLY DEVELOPMENT OF MAMMALS. 



chord may still continue to lengthen by acquisitions at its caudal end of additional 



cells from the primitive streak. 



After it is once formed as a band of cells, 

 the notochord passes through various changes of 

 form, but ultimately becomes a cylindrical rod with 

 tapering extremities. It attains" considerable size in 

 the embryos of most vertebrates, but in those of 

 placental mammals it is always small. It is prob- 

 able that in mammals the notochord, when first 

 separated from the entoderm, is a broad, flat band, 

 and that this band subsequently draws together, 

 diminishing its transverse and increasing its vertical 

 diameter until it has acquired a 'rounded form. 

 Finally its outline becomes circular in cross-section. 

 This series of changes begins near the anterior end 

 of the notochord and progresses both forward and 

 backward. 



In later stages the mes/)derm again grows 

 across the median line of the embryo, completely 

 surrounds the notochord, and forms a special 

 sheath about it. Still later the mesoderm forms a 

 broad envelope around the notochord, which we 

 can soon recognize as the anlage of the chondro style, 

 out of which the vertebral column and part of the 

 base of the skull are to be differentiated. Very 

 soon (Fig. 23) the chondrostylic anlage shows a 

 series of transverse discs of denser tissue, the 

 anlages of the intervertebral ligaments, the broader 

 light spaces between the discs being the anlages of 

 the vertebrae. In mammals, the notochord assumes 

 an undulating course, which may be slightly irregular 

 at first. The typical arrangement is shown in the 

 figure the dorsal summit of each flexure is in- 

 tervertebral, the ventral hollow of each flexure is 

 vertebral. 



FIG.33.-NOTOCHORDANDCHONDROSTYLE ^ UltimatC *** f the NotOChOrd. 



OF A SHEEP EMBRYO OF 14.6 MM. As the vertebral column develops, the notochord 



RECONSTRUCTION FROM SAGITTAL slowly disappears in the regions of the vertebra and 

 SERIES nog. SECTIONS IQO-IQ? ,1 



even the space occupied by it is obliterated by the 



growth of the body of the vertebra. In the intervertebral discs, on the contrary, 

 the notochord persists to form the nuclei pulposi of the adult. Each nucleus is 



