XIII 



PHYLUM CHORDATA 



127 



of the adult, so that there is no proetodseum ; the buccal cavity is 

 formed from a stomodaeal invagination at a comparatively late 

 period. 



.T 



int 



FIG. T5S. Petromyzon marinus. The uriuo-geiiital sinus with posterior end of intestine 

 and part of left kidney, a. anus ; int. intestine ; 1: left kidney ; '/-. rectum ; //. r/. />. urino- 

 genital papilla; c. </. .*. urine-genital sinus; <</. left ureter ; .<, ;/.', apertures of ureters into 

 urine-genital sinus ; //. bristle passed into right genital pore ; z, bristle passed from urino- 

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



mi.m 



en? 



FIG. 750. 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 ; F, G, two stages 

 in the development of the notochord and nervous system, bid. blastoccele ; hip. blastopore ; 

 <ct. ectodeiTii ; -/<f. enteron ; fix. spinal ganglia; A-. keel; nift. m. megameres ; mi. //<. micro- 

 meres; nch. notochord ; up. <<!, spinal cord. (After Shipley and Kupffer.) 



The formation of the nervous system is peculiar. The walls of 

 the medullary groove are in close apposition, so that the ill-growth 

 of ectoderm, from which the brain and spinal cord are developed- 



