74 



COMPARATIVE ANATOMY 



intact during this process of segmentation of the lateral folds, is the mate- 

 rial of the future notochord (Fig. 46, NC). 



At the time of hatching, then, the embryo has made important progress 

 beyond the gastrula stage. Not only has the segregation of mesoderm 

 begun but two important organs, the central nerve tube and the notochord, 

 are indicated. 



After hatching, additional pairs of mesodermal segments are cut off 

 from the lateral mesodermal folds, the addition taking place progressively 

 from anterior to posterior, until a total of ordinarily fourteen pairs have 

 been produced. In several of the more posterior segments cavities do not 

 occur, the mesodermal folds merely breaking up into a succession of soHd 

 blocks of cells. 



^MES 1-6^ e'c 



Fig. 47. — AMPHIOXUS. Frontal (horizontal) section of an embryo having 

 six pairs of mesodermal somites. The section is through the notochord and just below 

 the blastopore. At the posterior end of the section may be seen a region where the 

 notochord, endoderm and mesoderm merge indistinguishably. A, archenteron near 

 the blastopore; EC, ectoderm; EN, endoderm; MES 1-6, mesodermal somites; NC, 

 notochord. (After Cerfontaine.) 



In the formation of these fourteen pairs of mesodermal pouches the 

 material of the original mesodermal folds is completely utilized. These 

 folds, however, terminated posteriorly in the close vicinity of the blasto- 

 pore (Fig. 47) a region where, since an early gastrula stage, very rapid 

 cell proliferation has been going on. Following, then, the formation of 

 the fourteen pairs of mesodermal segments from the lateral folds, the series 

 of segments is extended backward by addition of successive soHd blocks 

 of cells which become detached from the growth zone encirchng the blasto- 

 pore. By this means the number of pairs of mesodermal units, the earlier 

 and more anterior developing as hollow endodermal outgrowths, the later 

 and more posterior as solid blocks derived by ingrowth from the blastoporal 

 rim, is increased to the adult total, usually sixty-one. 



In Amphibians. About certain details of mesoderm formation in 

 amphibians there is more or less obscurity. In some respects the process 

 differs conspicuously from that in Amphioxus. On the whole, however. 



