Plate XXI. 



a.cv., anterior cardiiial vein. 



OM., auditory pit. 



J)l)., ventral li]) of blasto[)ore. 



I>rg., brancliial region. 



bs., Llood space. 



ed., collecting duct. 



eh., chorda dorsali«. 



ce, cloacal cavity. 



c.dv., diverticuliun of cloacal 

 cavity. 



co.sd., cloacal opening of seg- 

 mental duct. 



cîo., wall of cloaca. 



cp., e[)iblast. 



(/L, glomerulus of proueplinjs. 



/'., dorsal tin. 



/(J., fore-gut. 



h., heart. 



hy., hypoblast. 



/., liver, or Anlage of liver. 



Lw., lateral plate of mesoblast. 

 mch., mesenchymatous cells. 

 7n.p., i)arietal layer of mesoblast. 

 perit., peritoneal membrane. 

 mt.T, II, &c., the first, second 



myotome, &c. 

 m.v., visceral layer of mesoblast. 

 n., neural canal. 

 nst.2-6, nephrostomes from the 



second to sixth. 

 pp.c, pleuroperitoneal cavity. 

 pjt. 2-5, pronephric tubules from 



the first to fifth. 

 r.m., radix of mesentery. 

 sell., subchorda. 

 sd., segmental duct. 

 t.a, tract of aorta. 

 t.ac. tract of anterior cardinal 



vein. 

 tr.a. truncus arteriosus. 



Figs. 98-106. From a series of transverse sections through an embryo of 



Stage V. 

 Figs. 107-110. From a series of transverse sections through an embryo 



of Stage VI. 

 Fig. 111. Transverse section through the cloacal region of an older embryo 



of Stage VI. 

 Figs. 112-114. A series of sagittal sections through a younger embryo in 



Stage IV. 

 Fig. 115. A. frontal section through an embryc» a little more advanced 



than that of Stage vi. 



