PHYLUM CHORD AT A 



127 



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

 formed from a stomodseal invagination at a comparatively late 

 period. 



!'[<.. T.>. Petromyzon marinus. The urino-geiiital sinus with posterior end of intestine 

 and part of left kidney, a. anus ; 'int. intestine ; 1:. left kidney ; r. rectum ; v. rr. p. urino- 

 genital papilla; i>. <j. s. urino-genital sinus; v.r. left ureter ; x, x',< apertures of ureters into 

 urino-genital sinus ; ?/, bristle passed into right genital pore ; ., bristle passed from urifio- 

 genital aperture into sinus. (From Parker's Znotomi/.) 



enf 



Fie. 7V.i. 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. blastocoele ; blp. blastoppre ; 

 ?ct. ectoderm ; ent. enteron ; gn. spinal ganglia ; k: keel ; mg. in. megameres ; mi. m. micro- 

 meres ; itch, notochord ; xp. c<l. 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 in-growth 

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



