REPRODUCTION 



55 



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 solid blocks of cells. (Fig. 42G) 



In the formation of these fourteen pairs of mesodermal pouches 

 the material of the original mesodermal folds is completely utilized. 



NP\ ,NC 



Fig. 44. — Sectiuns of an amphibian embryo at an early stage in the development 

 of the notochord and mesoderm. Semi-diagrammatic. A , median longitudinal section. 

 B, transverse section near the middle of the longitudinal axis. A, archenteron; BP. 

 blastopore; EC, ectoderm; EN, endoderm; MES, mesoderm; NC, notochord; NP, neural 

 plate. 



Later the series of segments is extended backward by addition of successive 

 solid blocks of cells which become detached from the growth zone encircling 

 the blastopore (Fig. 43). By this means the number of pairs of mesoder- 

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



In Amphibians. In amphibians, as in Amphioxus, the blastoporal 

 rim or germ-ring is the all-important source of mesoderm. The amphib- 

 ian, however, gives little evidence of anything comparable to the paired 

 mesodermal pouches which push out from the dorso-lateral endoderm of 

 Amphioxus. 



During the process of gastrulation in the amphibian the material 

 destined to become mesoderm lies within the advancing edge of the over- 

 growing fold (Fig. 40) which is the chief agency in the enclosing of the 

 yolk. As the edge of this fold, the narrowing blastoporal rim, advances, 

 it (in effect) leaves behind it — "behind" being anterior because the fold 

 advances posteriorly — a trail of potential mesoderm which, however, is 

 at first in no way distinguishable from other material destined to be 



