XIII 



PHYLUM ClIORDATA 



in the testis of the male. The repro(hictivo products arc shed 

 into the aflonie and make their way by the genital pores into 



Fi(i. 802. — Petromyzon marinus. The urinos^onital sinus with posterior end of intestine 



and imrt i if left kidney, it. :uius ; in(. intestine ; i-. left kidney ; >•. reetutn ; u.i/./i. urino 

 genital jiapillii ; <'.;/.»■. urinuiienital sinus; ur. left ureter; x, x', apertures of ureters into 

 urinogcnital sinus ; u, bristle passed into right genital pore ; z, bristle passed from urino- 

 genital aperture into sinus. (I'Yom Parker's Zootomy.) 



the urinogenital sinus, and so to the surrounding water, where 

 impregnation takes phice. 



Development. — The oosperm is telolecithal, having a con- 

 siderable accumulation of yolk in one hemisphere ; in correspon- 

 dence with this, segmentation is complete but unequal, the morula 

 consisting of an upper hemisphere of small cells or micromeres 

 (Fig. 808, mi. on.), free from yolk, and of a lower hemisphere of 



Tni.Tn 



Fig. 803. --Petromyzon. A and B, two stages in segmentation ; C, early embryo from the 

 posterior aspect ; D, section of blastula stage ; E, section of gastrula stage, hli). blastoiaore ; 

 hi. ct. blastocuele or segmentation-cavity ; k: keel ; niff. m. megameres ; mi. w. micromeres. 

 (After Shipley and Kupfier. ) 



large cells or megameres (mg.m.), containing much yolk. In the 

 blastula stage (D) the segmentation-cavity or blastocoele (bid.) is 

 situated nearer to the upper than to the lower pole. A trans- 

 verse semilunar groove appears which is bounded by a prominent 



