Embryonic Development; Differentiation of Tissues - 279 



lining throughout the digestive tract and all 



of its branches, and the mesoderm give rise 

 to all other body structures — such as the 

 muscle, bone, and connective tissues of the 

 animal. Animals possessing only ectoderm 

 and endoderm are said to be diploblastic, 

 whereas those having all three germ layers 

 are said to be triploblastic. 



The mesoderm arises differently in differ- 

 ent embryos. In the earthworm and many 

 other invertebrates, the mesoderm derives 



at or about the time of gastrulation (Fig. 15- 

 9). These mesoderm cells multiply rapidly, 

 forming two strands of tissue that crowd 

 forward on either side of the archenteron, 

 between the ectoderm and the endoderm 

 (Fig. 15-9). As the mesoderm proliferates, it 

 tends to fill and obliterate the blastocoel, en- 

 croaching above and below the archenteron 

 (Fig. 15-9). Then a new cavity develops in 

 the mesoderm itself; and this coelom, or body 

 cavity, is found in all higher animals. The 



chiefly from a pair of large cells, the pole coelom enlarges and encircles the archenteron 

 cells, which are pushed into the blastocoel completely, separating the mesoderm into 



GASTRULA 



BLASTOCOEL 

 ECTODERM 



POLE CELL 



BLASTOCOELn 

 ECTODERM- 

 ENDODERM 



POLE CELL 



FORMATION OF 

 MESODERM FROM 

 THE POLE CELLS 



FORMATION OF 

 COELOMIC CAVITIES 



ANUS 



MESODERM 

 POLE CELL 



COELOM 1 SEPTUM 



NERVE CORD 



Fig. 15-9. Development in the earthworm. Figures on the left represent horizontal 

 longitudinal sections, those on the right cross sections through the points indicated by 

 the dotted lines. 



