XIII 



PHYLUM CHORD ATA 



135 



in the testis of the male. The reproductive products are shed 

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



Fir;. 802. Petromyzon marinus. The urinogenital sinus with posterior end of intestine 

 and part of left kidney, a. anus ; int. intestine ; L: left kidney ; r. rectum ; u.ij.p. nrhio 

 genital papilla; u.<i.s. urinogenital sinus; ur. left ureter; x,x', apertures of ureters into 

 urinogenital sinus ; y, bristle passed into right genital pore ; z, bristle passed from urino- 

 genital aperture into sinus. (From Parker's Zootomy.) 



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

 impregnation takes place. 



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. 803, mi. m.), free from yolk, and of a lower hemisphere of 



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. Up. blastopore ; 

 t>l. cl. blastocuile or segmentation-cavity ; k. keel ; mg. m. megameres ; mi. m. micromeres. 

 (After Shipley and Kupffer.) 



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

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

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

 verse semilunar groove appears which is bounded by a prominent 



\ 



