260 



General Biology 



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

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

 large cells which grow as buds from the entoderm, and which, when 

 once formed, grow rapidly, forming the so-called mesodermic bands. 

 These bands later become hollowed out. The two cells which form 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 both these methods of mesoderm forma- 

 tion 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 ( ). 



Typhlosote 



Dorsal vessel 



Cuticle, 

 -Epidermis 

 Circular muscle 



•Longitudinal 

 muscle 



Nephroiome, 



SbicLQ/ 



Coelom 



flayer of 

 chlorogogen colls 



setae, 



Verve-cord 



Veniral vessel 

 K Submural vessel 

 Nkphrosiome 



Eig. 162. 



Transverse section through the middle region of the body 

 of the earthworm, Lumbricus. (From Parker and Haswell, 

 after Marshall and Hurst.) 



Fig. 163. 



Two stages in the early develop- 

 ment of a common fresh-water mol- 

 lusc, Planorbis, to show the origin 

 of the mesoderm cells. 



The ectoderm cells are deeply 

 shaded, the endoderm cells _ are un- 

 shaded. A. Young stage in which 

 the endoderm has not begun to be 

 invaginated; it is a lateral optical 

 section. B. Older stage, optical sec- 

 tion seen in front view; the endo- 

 derm 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 meso- 

 derm 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 greater 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. 



