INTRODUCTION TO THE COELOMATA 



259 



This mesoderm forms in two ways (1) either by little pouches grow- 

 ing from the entoderm which are then nipped off, or (2) by two large 

 cells which grow as buds from the entoderm, which once formed, grow 

 rapidly, forming the so-called mesodermic bands, which bands later be- 

 come hollowed out. The two cells forming the original bud are termed 

 pole-cells. This hollowed out portion is the coelom. A close study of 

 Figure 163 will make a better understanding of the above possible. 



It must be understood that these two forms are not likely to be found 

 in any one animal. 



The open space thus formed which we have called the coelom has 

 thus a layer toward the outside of the body and a layer of cells or wall 

 toward the entoderm from which it sprang. The outer wall of the coelom 

 is called the somatic layer or the somatopleure ( ), 



while the inner is known as the splanchnopleure ( ). 



hep 



JZtrvph 



Jiephrost 



Fig. 162. 



Transverse section through the middle 

 region of the body of the earthworm, Lum- 

 bricus, circ.mus, circular muscle fibers ; coel, 

 coelom ; dors.v, dorsal vessel ; epid, epidermis ; 

 ext.neph, nephridopore ; hep, chlorogogen cells ; 

 long.mus, longitudinal muscles ; neph, nephri- 

 dium ; nephrost, nephrostome ; n.co , nerve- 

 cord ; set, setae ; sub.n.vess, subneural vessel ; 

 typh, typhlosole ; vent.v, ventral vessel. (From 

 Parker and Haswell, after Marshall and 

 Hurst): 



Fig. 163. 



Two stages in the early development of a 

 common fresh-water mollusc, Planorbia, to 

 show the origin of the mesoderm cells. 



The ectoderm cells are deeply shaded, the 

 endoderm cells are unshaded. A. Young stage 

 in which the endoderm has not begun to be 

 invaginated ; it is a lateral optical section. 

 B. Older stage, optical section seen in front 

 view ; the endoderm cells are invaginating, 

 and the two mesoderm cells are seen on each 

 side. 1. Mesoderm or pole-cells ; in B, each 

 has budded off another mesoderm cell. (After 

 Rabl). 



When pole-cells form, the cavity of the digestive canal is small in 

 proportion to the thickness of its wall, so that the pole-cell may be con- 

 sidered as "a solid pouch." 



In most Coelomata the mesoderm or coelomic wall forms by far the 

 greatest portion of the body. There are sometimes cells which form in 

 the primary body-cavity, to which some writers have also applied the 

 term mesoderm. This term should, however, be reserved for the walls 

 of the coelom as just described, while mesenchyme ( ) 



should be used for the cells forming within the primary body-cavity. 



Mesenchyme arises from different germ-layers in different phyla of 



