PHYLUM CIIOKDATA 



l:!.i 



in the testis of tho iiialo. Tlio reproductive pn.iduets arc shed 

 into the codume and niike theii w \\ 1>} thi genital pores into 



«■?•? ^ 



Fn;. S0_',— Petromyzon marinus. Tlu- uriuogunital siima with itnstcriur end of iiitcatino 

 and part of left kidney, a. anus; int. intestine: Ic. left kidiiej' : r, reetnin : I'.ii.ji. nriini 

 genital papilla; n..i/.s. urinogenital sinus; (';■. left ureter; ,.',.''', ajtertures tif ureters into 

 urinoyenital sinus ; i/, bristle passed into right genital pore; r, Ijristle passeil frnin urinn- 

 genital aperture into sinus. (i''runi Parker's Zooloui//.) 



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



,,<5//) 



>inyZon. A and B, two stages in segmentation; C, early embryo from the 

 eet ; Ll, section of blastula stage ; E, .seetion of gastrula stage, hi/,, blastopore; 

 ceele or segTiientation-cavity ; I: keel; iiuj. m. megameres ; ml. „<. mieronieres. 



11. sos.-Petromyzon 

 posterior aspee 

 hi. cL blastoeoile or segine 

 (After Shipley and Kupffer.) 



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

 blastula stage (D) the segmentation-cavity or blastocojle (llcl.) is 

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

 verse semilunar groove appears which is bounded by a prominent 



