EXPLANATION OF PLATE Xllf. 



The dorsal part of the central compartment of the body cavity has 

 increased in size, but is still separated ventrally by a septum. The 

 ventral blood-spaces have increased in size. The ventral division of 

 the coelom is assuming the form of a nephridium. 

 Fig. 17. Diagram of arrangement at birth. The two halves of the peri- 

 cardial cavity have coalesced dorsally and ventrally to the median- 

 dorsal blood-space which forms the heart. The dorsal divisions of tiie 

 coelom have become constricted off from the floor of the pericardium 

 and the dorsal wall of the enteron, and now lie in the central compart- 

 ment of the body-cavity and constitute the generative tubes (in the 

 anterior part of the body they atrophy in the floor of the pericardium). 

 The horizontal septum separating the dorsal division of the central 

 compartment of the body cavity from the ventral has vanished. Tiie 

 ventral or nephridial division of the coelom has assumed the form of a 

 nephridium. On the left hand the whole course of the nephridium is 

 diagrammatically shown. 



