ANATOMY OP THE MADREPORARIAN CORAL PUNGIA, 315 



all those forms in which a coelom is recognised the division 

 of the mesoblast into splauchuopleure and somatopleure, aud 

 its relation to the coelom in limiting it^ must enter largely into 

 our conception of what is meant by the term mesoblast. 



Tiie origin of the mesoblast is very various ; for information 

 on this point I must refer the reader to Balfour's 'Comparative 

 Embryology/ vol. ii, p. 290^ where a tabular account of its 

 various modes of origin is given. From this table it will be 

 seen that while instances of a mixed derivation of the meso- 

 blast are not common, a purely epiblastic derivation is still 

 more uncommon, occurring in fact only in the larva of Desor, 

 Bouellia, and perhaps in Lumbricus trapezoides. A 

 purely hypoblastic derivation is of frequent occurrence. It 

 is generally admitted that part of the mesoblast, at any rate, 

 was primitively derived from the epiblast ; that in many forms 

 all traces of this derivation are lost has been explained by 

 Lankester (26 and 27) by his theory of precocious segregation. 

 On the other hand there is much evidence in favour of the view 

 that the coelom is derived from archenteric diverticula, and 

 that the limiting walls of the coelom are in consequence de- 

 rived from hypoblast. This is clearly the case in several 

 groups ; in others there is reason to believe that the origin of 

 the mesoblast as ingrowths from the lips of the blastopore is 

 an abbreviation of development, and that in the ancestors of 

 the groups in which this occurs the mesoblast took its rise 

 from the walls of outgrowths of the archenteron. 



It is assumed that the triploblastic Metazoa took their origin 

 from the diploblastic Metazoa, as the Coelenterates have been 

 called (1 leave the Dicyemidse and Orthonectidse out of the 

 question) . The Coelenterate, represented by an Actinia, already 

 in the elongation of its mouth and the arrangement of its 

 mesenteries, shows a tendency to bilateral symmetry. It is 

 supposed that this tendency is further increased, that the radial 

 symmetry of the peripheral chambers is replaced by a bilateral 

 symmetry, metamerically repeated along the long axis formed 

 of the arclieuteron. If this were admitted it would admit the Turbellaria 

 among the Coelomata. 



