iv. 8- 



96 VERTEBRATES WITHOUT JAWS 



these ridges and develop into eggs or sperms. The differentiation of 

 the gonad occurs relatively late in lampreys, so that in young am- 

 mocoetes the organ is 'hermaphrodite', containing developing oocytes 

 and spermatocytes together. The ripe ovary consists of ova each sur- 

 rounded by single-layered follicular epithelium, which finally ruptures 

 and liberates the egg into the coelom, whence it escapes by pores to be 

 described presently. The testis consists of a number of follicles con- 

 taining sperms; it is unique among vertebrates in that the follicles 



CL U 9 . 



Fig. 62. Cloacal region of fully adult Lampetra. 



C. coelom; CI. lips of cloaca; Ct. connective tissue; D. duct leading from coelom to the 



mesonephric duct; Df. dorsal fin; M. muscle; Md. mesonephric ducts; N. notochord; 



R. rectum; Ug. urinogenital papilla. (After Knowles.) 



have no ducts; when ripe they rupture into the coelom, which becomes 

 filled with spermatozoa and these escape, like the ova, by pores. 



These apertures by which the gametes escape are similar in the 

 two sexes and consist of short channels, one on each side, leading from 

 the coelom to the lower end of the kidney duct (Fig. 62). They nor- 

 mally become open only a few weeks before spawning, but Knowles 

 has shown that injections of oestrone or anterior pituitary extract will 

 cause perforations of the ducts in young lampreys, indeed even in the 

 ammocoete larve. 



Fertilization is external, but there are modifications of the cloaca in 

 the two sexes to assist in ensuring fertilization and proper placing of 

 the eggs in the 'nest' (p. 113). The lips of the cloaca of the ripe male 

 are united to form a narrow penis-like tube. The cloacal lips of the 



