570 



CEPHALOCHOKDA. 



connected with this wall by means of a stalk-like prolongation of the 

 epithelium. Consequently the genital gland does not originate in the 



segment in which it lies later, but is derived 

 from the anterior wall of the segment im- 

 mediately following. 



Fig. 308 shows a transverse section of 

 the genital rudiment approximately at the 

 stage represented in Fig. 307. The fusion 

 of the rudiment with the inner wall of the 

 somite is visible, a connection which is per- 

 manently retained. The genital rudiment, 

 at this stage, still lies in the myocoele. 

 Figs. 309 and 310 illustrate the way in 

 which the part of the myocoele which cor- 

 responds to the genital rudiment becomes 

 severed to form a distinct genital chamber. 

 This takes place through the formation of 

 a fold (w) which grows backward from the 

 anterior apex of the genital pouch till, finally, 

 in individuals 16 mm. long, the communication between the genital 

 chamber and the myocoele is entirely lost. The genital pouches 

 at first corresponded merely to the posterior angle of the myocoele 

 to which they belonged, but they extend so far forward secondarily 

 that each chamber comes to lie in contact with the posterior end of 

 the one next in front of it. 



FlG. 308. — Transverse sec- 

 tion through a stage 

 corresponding to that 

 depicted in Fig. 307 (after 

 Boveri). bm, ventral 

 muscle; .'/. lilood-vessel ; 

 gd, genital gland. 



Fig. 309.— Two consecutive genital vesicles oi an Amphioxus 13 mm. long (after 

 Boveri). 



Si. as to complete our description of the organisation of A-nij>Icio.rvx 

 we must, in conclusion, briefly describe the renal canals discovered by 

 Boveri (Nos. 1 and 2) in the adult animal. These are short tubes 

 found in the region of the pharynx which connect the sub-chordal 

 coelom (Fig. 311, sc) with the atrial cavity (p). They lie on the 



