56 OUTLINES OF CHORDATE DEVELOPMENT 



gonadial cavities which appear later have been compared 

 with the nephrostomes, or "gono-nephrostomes," of the higher 

 forms, and it seems likely that Amphioxus may be primitive 

 as regards the position and origin of the germ cells. At any 

 rate Amphioxus is primitive in that the gonads arise and 

 remain as metameric structures, entirely separate from the 

 excretory system. 



Shortly after the completion of the larval period, in specimens 

 about 5 mm. long, these small groups of cells are found in the 

 ant ero- ventral region of the segment, toward its inner or atrial 

 surface, lying along the posterior face of the dissepiment sepa- 

 rating the surrounding myocoel from that next anterior. As 

 these cells slowly multiply they push forward into this next 

 anterior myocoel forming, in its postero-ventral region, a small 

 bud covered with a fold of the dissepiment. This dissepiment 

 soon becomes sac-like and remains attached by a short stalk 

 to the anterior face of the remainder. This solid gonadial rudi- 

 ment soon develops a cavity within its mass. When the larva 

 is about 12 mm. long, this general region of the myoccel becomes 

 cut off by a fold the outer myotomal wall, leaving the gonad 

 surrounded outside of its own wall by a portion of the general 

 myocoelic space; this is the perigonadial coelom or gonocoel (Fig. 

 21, A). In the base of this fold is a blood-.vessel a branch of 

 the posterior cardinal vein, which extends through this entire 

 region. 



Then the gonadial cells move toward the atrial side of the 

 original gonadial sac, leaving the dissepiment al wall as the 

 visceral wall of the gonocoel while its parietal wall is formed 

 by this latter downgrowth of the myotomal wall (Fig. 21, B). 

 The cavity of the gonad enlarges, as the primary gonadial 

 (ovarian or testicular) cavity, toward the outer or atrial side of 

 which the definitive germ cells are crowded and become covered 

 by a follicular epithelium, also formed from cells of the original 

 mass. The later history is more completely w r orked out in 

 connection with the ovary, although it is known that the 

 development of the testis is closely similar. 



The outer (atrial) region of the gonocoel, lying between the 



