68 THE EARLY DEVELOPMENT OF MAMMALS. 



extremity of the notochord until somewhat later, so for a considerable period 

 the cephalic end of the notochord remains fused witlrthe entoderm. The sepa- 

 ration from the entoderm is effected in mammals by the entoderm proper shoving 

 itself under the notochord toward the median line. When the cells from one side 

 meet, those of the other are united with them and form a continuous sheet of 

 entoderm below the notochordal cells. " 



After its separation the notochord is a narrow band of cells, which starts 

 anteriorly from the entoderm (the future lining of the alimentary tract), running 

 backward to the blastopore. So long as the blastopore is open the notochord 

 terminates in the epithelium lining it. For a certain period the notochord con- 

 tinues to grow tailward by accretion of cells from the walls of the blastoporic 

 passage; and after the canal is permanently obliterated, the notochord 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 probable 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 for- 

 ward and backward. 



In later stages the mesoderm 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 around the notochord groups of cells which 

 we can identify as the anlages of the vertebrae and of certain parts of the skull. 



The Ultimate Fate of the Notochord. 



As the vertebral column develops the notochord degenerates. It first ceases 

 to be continuous by breaking apart at points corresponding approximately to 

 the center of each vertebra. The fragments of the notochord contract and form 

 little masses situated in cavities in the intervertebral spaces. These cavities 

 have a distinct boundary and present characteristic forms in different mammals. 

 The notochordal cells do not fill the cavity. The sheath of the notochord is lost ; 

 the walls of the cells disappear ; the tissue becomes granular and breaks up into 

 multinucleate, irregularly reticulate masses which are gradually resorbed (Fig. 

 24). Tissue of this character may be easily observed in human embryos of the 



