Reproduction 261 



V MES 1-6' 



Fig. 214. Arnphioxus. 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. Courtesy, Neal and Rand: "Chor- 

 date Anatomy," Philadelphia, The Rlakiston Company.) 



material of the original mesodermal folds is completely utilized. Later 

 the series of segments is extended backward by addition of successive 

 solid blocks of cells which become detached from the growth zone 

 (germ-ring) encircling the blastopore (Fig. 214). By this means, the 

 number of pairs of mesodermal segments is increased to the adult total, 

 usually 61. 



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

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

 phibian, however, gives little evidence of anything comparable to the 

 paired mesodermal pouches which push out from the dorsolateral en- 

 doderm of Arnphioxus. 



During the process of gastrulation in the amphibian, the material 

 destined to become mesoderm lies within the advancing edge of the 

 overgrowing fold (Fig. 211) 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 be- 

 cause the fold advances posteriorly — a trail of potential mesoderm 

 which, however, is at first in no way distinguishable from other mate- 

 rial destined to be permanently endoderm (Fig. 211C, EN). That is, 

 the two materials together and in no way delimited from one another 

 constitute the deeper layer of the overgrowing fold. Later this layer 

 virtually splits (the process called "delamination") to form two layers, 

 an inner one abutting on the archenteric cavity and an outer one which 

 is then recognizable as a definite mesoderm (Fig. 215B). This layer, 

 although now distinct from the endoderm which parallels it, for a time 

 retains continuity with its source, the proliferating zone about the 



