The pronephros of Chrysemys marginata. 49 



of a portion of the lateral plate region of the general coeloni, and 

 so the tubule openings instead of being coelomic become nephrotomic. 



The bearing of these changes upon the kidney ontogeny is dis- 

 cussed in the portion of this paper dealing with theoretical con- 

 siderations. 



The development of the lumen in the pronephric duct takes 

 place from before backwards; in an embryo with 18 protovertebrae 

 the lumen has extended back to a region a little behind the XVIII *^ 

 protovertebra ; in an embryo with 23 protovertebrae the lumen is 

 found as far back as protovertebra XXI; and in an embryo with 

 26 protovertebrae the lumen extends to about segment XXIII. 



Thus the evidence of the pronephric duct development in 

 Chrysemys points to a homodynamous urinary organ, the anterior 

 part of which is pronephros and the succeeding part mesonephros, 

 the development proceeding similarly and without break from before 

 backwards, the tubules suffering a more arrested development the 

 further back along the urinary organ we pass. 



The blood supply of the developing kidney in 

 Chrysemys marginata. 



Embryos A and B. 

 No dorsal aorta or blood-vessels to the kidney yet formed. 



Embryos C — H. 



Dorsal aortae present and extending laterally outwards to the 

 splanchnic mesoderm (see Plate 1 and 2 Fig. 14, 15. 16. 17). 



The dorsal aortae seem to extend out lateralwards into the 

 nephrotome region or into the region immediately beneath the 

 nephrotome. 



Embryo I. 



On the left side blood-vessels pass off from the dorsal aorta to 

 the kidney in sections: 



3,31—3,32 

 4,16 

 4,19 

 4,25 



Zool. Jahvb. XXXVI. Abt. f. Anat. 4 



