120 



ZOOLOGY 



SECT. 



a.s.c 



jy.s.e 



sac 



FIG. 750. Auditory sac of Petromyzon. 

 a. s. c. anterior semicircular canal ; and. n. 

 auditory nerve ; end. s. endolymphatic 

 sac ; p.s.c. posterior canal ; sac. sacculus ; 

 lit,-, utriculus. (After Retzius.) 



embryo. Each is attached along one edge to the dorsal wall of the 

 body cavity by a sheet of peritoneum ; along the other or free 



edge runs the ureter (ur.), which 

 is the undivided segmental duct. 

 The ureters open posteriorly into 

 a small urino-genital sinus (Fig. 

 758, u.g.8.), placed just behind the 

 rectum, and opening, by a urino- 

 genital papilla (u.g.p.\ into a pit 

 in which the anus (a) also lies. 

 The side-walls of the sinus are 

 pierced by a pair of small aper- 

 tures, the genital pores (y), which 

 place its cavity in communication 

 with the coelome. 



The gonacl (Fig. 752, ov, Fig. 



758, ts) is a large unpaired organ occupying the greater part of 

 the abdominal cavity and suspended by a sheet of peritoneum. 

 The sexes are separate, but ova have been found in the testis of 

 the male. The reproductive products are shed into the coelome 

 .and make their way by the genital pores into the urmogenital 

 sinus, and so to the surrounding 

 water, where impregnation takes 

 place. 



Development. The oosperm is 

 telolecithal, having a considerable 

 accumulation of yolk in one hemi- 

 sphere ; in correspondence with this 

 segmentation is complete but un- 

 equal, the morula consisting of an 

 upper hemisphere of small cells or 

 micromeres (Fig. 759, mi. m.), free 

 from yolk, and of a lower hemisphere 

 of large cells or megameres (mg. m), 

 containing much yolk. In the bias- 

 tula stage (D) the segmentation 

 cavity or blastocoele (bid.) is situated 

 nearer to the upper than to the 

 lower pole. The gastrula is not 

 formed by invagination (E), but a 

 cavity appears among the cells of 

 the upper pole and becomes the 

 archenteron, its aperture being the 

 blastopore (C and E, Up.}. The un- 

 symmetrical positions of the blastocoele and archenteron are due 

 to the comparatively rapid division of the micromeres as compared 

 with that of the inert yolk-cells. The blastopore becomes the anus 



cL.ao 

 cd 



int 



Fi<;. 757. Petromyzon marinus. 



Transverse section of abdomen. '/. 

 cardinal veins ; (/. no. dorsal aorta ; 

 f. r. fin-rays ; /. t. fibrous tissue of 

 spinal canal ; int. intestine, the line 

 pointing to the spiral valve ; k. 

 kidneys ; h/. sub-vertebral lymph 

 sinus ; in. body muscles ; //<//, spinal 

 cord ; nc. notochord ; n. ca. spinal 

 canal ; ts. testis ; ur. ureter. (From 

 Parker's Zootomy.) 



