192 OUTLINES OF CHORDATE DEVELOPMENT 



closed anteriorly and laterally, but posteriorly it is at first 

 directly, though incompletely, open into the general body 

 cavity or peritoneal cavity. During the early stages the liver 

 forms the hinder wall of the pericardial cavity, medially, but 

 it still remains open postero-laterally, either side of the liver, 

 and medio-ventrally, below it. When the ductus Cuvieri are 

 formed, passing from body wall to sinus venosus, they traverse 

 this region of the coelom and aid in establishing the hinder wall 

 of the pericardial cavity. Incomplete peritoneal folds from 

 the body wall accompany the ductus Cuvieri from the body wall 

 to the heart; these are known as the lateral mesocardia. Dor- 

 sally the lateral mesocardia remain incomplete for a long time, 

 but ventrally they gradually extend to the body wall entirely 

 across the coelom and form a complete ventral partition, be- 

 tween pericardial and peritoneal cavities. The transverse peri- 

 toneal fold thus formed is the pericardio-peritoneal septum, or 

 septum transversum. Its median ventral portion appears to be 

 formed by the peritoneum originally covering the anterior face 

 of the liver; this separates from the liver and becomes added 

 to the septum transversum. On the right side it becomes 

 continuous with the posteriorly directed suspensory fold of the 

 liver (mesohepaticum). Not until after metamorphosis is the 

 septum transversum fully united dorsally with the dorsal 

 mesentery, and the separation of the pericardial and peritoneal 

 cavities entirely completed. 



VI. THE URINOGENITAL SYSTEM 



The excretory and reproductive systems develop independ- 

 ently and at widely different times, but in their definitive 

 state they form a complex, in which structures orignally ex- 

 cretory, have assumed morphological and functional relations 

 with the reproductive system, while other parts function, at 

 different times, both as excretory and reproductive organs. 



1. The Excretory System 



A functional excretory system is already established, before 

 the rudiments of the reproductive system have more than made 



