iv COELOME 199 



ancestral Vertebrate was divided up into segmentally arranged 

 compartments by transverse septa, and into a right and left half by 

 a sagittally placed partition supporting the alimentary canal and 

 forming the dorsal and ventral mesentery; in other words that the 

 general arrangement was like that of a primitive Annelid worm. 

 This seems to be indicated by the mode of development of the 

 mesoderin in Amphioxus. 



In Vertebrates above Amphioxus the segmented condition of the 

 splanchnocoele has disappeared even from development. 1 The 

 sagittally placed mesentery on the other hand still appears in 

 ontogeny in the form of the partition remaining between the edges 

 of the lateral mesoderm as they approach one another on the ventral 

 and on the dorsal sides of the alimentary canal respectively. In 

 correlation with the great increase in length, and consequent coiling, 

 of the alimentary canal of the Vertebrates a condition which 

 probably existed even in the ancestors of those gnathostomes in 

 which the alimentary canal is now short (p. 184) the ventral 

 mesentery disappears at an early stage of development throughout 

 that portion of its extent which lies on the tailward side of the 

 liver. 



The dorsal mesentery on the other hand persists throughout life, 

 serving as a bridge to carry the complicated connexions of the gut 

 wall with the vascular and nervous systems, although perforations 

 may appear in it, more or less extensive in different groups of 

 Vertebrates. The complicated foldings and frillings which the 

 dorsal "mesentery undergoes, owing to its enteric edge having 

 to keep pace with the increase in length of the gut, are of 

 interest mainly to specialists in the anatomy of particular groups 

 and need not be dealt with here. 



In the fishes, in which the lung performs an important hydro- 

 static function, that organ grows back in the substance of the 

 dorsal mesentery, and in accordance with its tendency to 

 assume a more and more dorsal position, the portion of mesentery 

 lying above it may become incorporated in the dorsal wall of the 

 splanchnocoele, the result being that the lung in the adult now lies 

 entirely dorsal to and beyond the limits of the body-cavity (Dipnoi, 2 

 Actinopterygii). 



Apart from its primary segmentation, the splanchnocoele shows 

 a tendency for special portions to become secondarily separated off 

 from the main cavity. The most important case of this occurs at 



1 While it has to bo granted that the splanchnocoele of the Vertebrates represents 

 the ventral portion of the coelome which has lost its segmentation, care must be 

 taken not to assume that this loss of segmentation has necessarily extended dorsal- 

 wards to precisely the same level in all Vertebrates. Like other anatomical 

 boundaries the dorsal limit of the splanchnocoele i doubtless fluctuating and vague. 

 It is therefore wise not to attach too great importance to the exact position of the 

 first rudiment of an organ which develops in one case on the dorsal and in another 

 on the ventral side of the boundary between segmented and unsegmented mesoderm 

 such as for example the gonad (p. 270). 



8 Cf. Graham Kerr, 1910. 



