426 



AN INTRODUCTION TO ZOOLOGY 



nephrotome adjoining the somite become converted into a scattered 

 mesenchyme, and so connection with the somite is lost. By the 

 fourth day the pronephfic tubules have disappeared. 



The second kidney or mesonephros develops in the 

 segments from the thirteenth or fourteenth back to the thirtieth, 

 and some of them therefore overlap the pronephros. However, it 

 is only those in segments 20-30 that develop typically, and these 

 do not begin to appear until the fourth day. In this region the 

 nephrotomes separate from the somitesand form a continuous band 



A.C. 



S.M. 



Ex 



SpM 



FIG. 149. Chick embryo. Transverse section of embryo with thirty-five pairs 

 of somites (about 72 hours). 



A., amnion ; A.C., amniotic cavity ; C., chorion ; D., dermatome ; D.A., dorsal aorta ; E., 

 embryonic ccelom ; EC., ectoderm ; En., Entoderm ; Ex., exocoel ; M., mesonephric tubule 

 My., myotome ; N., notochord ; N.C., neural crest : beginning of spinal ganglion ; P., post-cardinal 

 vein ; S., sclerotome ; S.A., sero-amniotic junction ; S.C., sub-cardinal vein ; S.M., somatic 

 mesoderm ; Sp.C., spinal cord ; Sp.M., splanchnic mesoderm ; V., vitelline artery ; W., Wolffian 

 duct. 



of tissue lying between the somite, the lateral mesoderm, the 

 dorsa aorta and the Wolffian duct. Dorsal to this region at quite 

 an early stage a venous trunk, the posterior cardinal vein, is laid 

 down. Each tubule first appears on the ventral side of the nephro- 

 tome as a group of cells which soon acquire a hollow, and so is 

 vesicular from the commencement. This vesicle becomes elongated 

 and bent, and one end acquires an opening into the Wolffian duct, 

 while the other enlarges to form the rudiment of a Bowman's capsule. 

 Later, secondary capsules to the number of four or five arise in each 

 somite, and altogether they form quite a noticeable swelling on the 



