134 INTRODUCTION TO THE COELOMATA [CH. 



or space is to describe it negatively : with as much justice. the endo- 

 dermic tube might be described as a split. In each case the real 

 object of consideration is the wall, and this is the point where the 

 coelom which the Germans appropriately name the secondary body- 

 cavity differs fundamentally from the primary body-cavity, for the 

 outer wall of the latter is merely the ectoderm. In many animals 

 such as Mollusca the two types of body-cavity coexist, and indeed 

 in all the higher animals the vessels or tubes which convey the 

 blood may be said to be remnants of the primary body-cavity. 



ABC 



FIG. 57. Three transverse sections through a developing Amphioxus to show 

 origin of mesoblast from endodermal pouches x 435. From Hatsehek. 



The ectoderm is deeply shaded, the mesoderm is lightly shaded, the endoderm 

 alimentary canal and notochord is unshaded. A. shows the origin 

 of the paired mesodermal pouches from the archenteron ; the cavity 

 coelomic of the former is still in communication with the cavity of the 

 alimentary canal. The notochord is arising in the middle line from the 

 endoderm, and the tubular nervous system above it is already separated 

 from the ectoderm. B. shows the mesodermal pouches completely shut off; 

 they each enclose a cavity, the coelom, and each consists of an outer wall 

 next the ectoderm, the " somatopleure," and an inner wall next the endoderm, 

 the " splanchnopleure." C. shows the meso dermal pouches extending ven- 

 trally beneath the notochord, now completely separated from the wall of 

 the alimentary canal and also round the alimentary canal. The coelomic 

 space is larger, and the splanchnopleur is beginning to form muscle-cells. 



If we leave out of account cases in which the facts of develop- 

 ment have not been fully elucidated and confine our attention to 

 those instances where the whole history of the coelom has been 

 exhaustively worked out, we find that this important organ arises 

 in one of two ways, either (l) by the formation of pouches of 

 the endodermic tube, which become nipped off (Fig. 57) ; or (2) by 

 the proliferation of two large cells, formed themselves by budding 

 from the endoderm (Fig. 58), which subsequently grow rapidly 

 and divide so as to form bands, the so-called mesoderm ic bands, 

 these bands later become hollowed out. These initial cells are 

 termed pole -eel Is. 



