154 



OF AMPHIOXUS. 



The gonadic pouches next become gradually constricted 

 off from the myoccelic spaces, and eventually lose all com- 

 munication with them. In the midst of the at first solid 



mass of primitive sexual 

 cells a cavity subsequently 

 appears, and the gonad be- 

 comes a hollow sac (Fig. 89). 

 In the course of its fur- 

 ther growth the gonadic sac 

 (not to be confused with the 

 gonadic pouch in which it 

 lies) grows out into a num- 



Fig. 89. Portion of transverse sec- i f i i i 



tion through an individual of 13 mm., Der lappets, and SO D6- 



to explain the conditions observed in CO meS a racemose reproduc- 

 preceding preparation. (After BOVERI.) 



b.v. Blood-vessel, go. Gonadic sac. tive gland (Langerhans). 



p.g.c. Perigonadial ccelom (sonadic .-pi ... , ,, 



pouch). ^.Transverse muscles. The The primitive SCXUal cells 



index line to which there is no letter remain for a considerable 



indicates the fold by which the gonadic 



pouch becomes constricted off from the length of time in an absO- 



lutely indifferent condition, 



and it is impossible to distinguish the male from the 

 female. 



According to LANGERHANS, sexual differentiation does 

 not begin to take place until the individuals have reached 

 a length of 17 mm., and sometimes it does not occur until 

 a much later period. It is inaugurated by the commence- 

 ment of the processes of spermatogenesis and ovogenesis. 

 There are no accessory sexual characters in Amphioxus, 

 and the sex can only be determined by an examination of 

 the reproductive glands. 



The segmental arrangement of the formative centres 

 of the reproductive organs at the base of the myotomes 

 is again met with in the embryonic development of the 

 Selachians, as shown by RUCKERT (Fig. 90). Here, also, 



