SUBKINODOM METAZOA. 57 



side coming iuto contact above and below, the united walls 

 forming the dorsal and ventral mesenteries which suspend the 

 intestine (Fig. 27, am and bm). That wall of each sack which 

 surrounds the digestive tract is termed the splanchnic layer of 

 the mesoderm (Fig. 27, spm), while that lying immediately 

 below the ectoderm is the somatic layer {sm), and the enclosed 

 cavity is the ccelom (0) or body-cavity. In other cases the 

 protoplasm destined to give rise to the mesoderm segregates 

 into a small number of cells, or sometimes even into a single 

 cell, at an early period of the development, frequently while 

 the embryo is still in what may be considered the blastula 

 stage. These cells, known as mesohlasts, give rise by repeated 

 division in one direction, and by the subsequent division of 

 the daughter cells so formed, to bands of mesodermic tissue 

 extending along the ventral surface of the embryo (see Fig. 

 105), and later growing dorsally so as to enclose the diges- 

 tive tract. The coelom forms by the hollowing out of the 

 mesodermic bands, and when fully developed presents the 

 same appearance as in the former case. 



In many animals, such as some Turbellarian worms, a 

 well-developed coelom is not present, the only traces of it 

 being minute scattered cavities in a mass of mesodermic tissue 

 which fills up the space between the endoderm and ectoderm. 

 A strict demarcation of this form of coelom (schizoccel) from 

 the other variety (enteroccel) does not, however, exist, grada- 

 tions occurring in various groups of animals and both varie- 

 ties sometimes being coexistent in the same form, as for 

 instance in bivalve Mollusca, where the pericardial cavity is 

 to be regarded as an enteroccel, while the spaces existing else- 

 where in the mesoderm are schizocoels. 



If the conditions which exist in the lowest triploblastic animals known 

 to us, the Turbellarian worms, throw any light upon the origin of the meso- 

 derm, it would seem that primitively it was a solid tissue, not completely 

 marked off from the endoderm, and that any coelom that it contained was 

 of the nature of a schizocoel. From this condition it became more and 

 more difEerentiated from the endoderm proper, and either tended to appear 

 as a separate germ-layer at an early stage of development in the form of 

 the mesoblasts, or was delayed in its development until after the formation 

 of the primitive digestive tract, from which it then separated in the meso- 

 dermic pouches. According to this view the mesoderm is a secondary 



