568 



CEPHALOCHORDA. 



fold-cavities. The cavity of the upper fold (of) lies in the meta- 

 pleural folds and is also known as the metapleural cavity. In its 

 inner wall, which is in contact with the outer wall of the atrial cavity, 

 the transverse or sub-atrial muscle (mt) develops. The cavities of 

 the lower folds (uf) are regarded by Hatschek as the equivalent 

 of the ventral fin-cavities in that part' of the body which lies behind 

 the atriopore. The splanchnocoele is divided by the gill-clefts into an 



SIX sx 



Fig. -jO."!. — Side view of the lower edges of the proto-vertebrae in a young Amphioxus, 

 9 mm. long (after Boveri). hm. ventral muscle ; gd, genital gland ; S IX, S X , 

 ninth and tenth mesodermal somites. 



upper paired cavity (■•>/■, epi-branchial , suyra-pharyngeal or sub-chordal 

 eoelom) and an unpaired ventral cavity (ec, mdostylar coelom). These 

 two are connected by means of canals running within the gill-bars 

 (cf. p. 556 and Fig. 311, on the right side of which a primary gill-bar 

 with its coelomic canal is represented as cut through longitudinally). 

 The development of the genital organs has recently been described 

 by Boveri (No. 3). The genital vesicles which develop in each 



FIG. 304.— Hide view of the 

 (alter Boveri). 



jenita 



urn. long 



mesodermal somite from the tenth to the thirty-fifth, are abstricted 

 portions of the somites which may be compared to the nephrotomes 

 or gononephrotomes (Ruckert) of Selachian embryos. Boveri, 

 therefore, regards the genital chambers of Amphioxus as the homo- 

 logues of the canals of the primitive kidney in the Craniata. The 

 development of these chambers can be observed in the quite young 

 Amphioxus four to twelve millimetres long. In cross-sections, at the 



