EXl'LANATION OF THE I'LATES. xiii 



around tlie trunks of tlic onipUalo-mesenteric vcBsels ; thift is 

 the postc'iior part of the luteial mass of l)lastt'nia marked d 

 in Fi;,'. 1, and becomes in the coui"se of development the 

 sujira-renal capsule. 

 Fig. 3. An early embryo of tlie sheep. 



a. Head, branchial arches, and rudiment of the eye. 



b. Heart. 



c. Ductus Cmaeri entering the auricle, and receiving 



d. The jugular, and 



e. The cardinal vein. 



/. The lateral bhtstenia. 



<j. Wolffian bodv. 



/t. Unjbilical cord, to which is pa.ssing 



?'. The allant(jis. 



j. The omiihalo-mescnteric artery, and 



/•. Oniphalo-mesenteric vein ; traces of the umbilical vessels are 



also iHicn in tlie parietes of the abdomen. 

 I. Tlie liver and inte.-^tiual tube. 

 m. Lungs. 

 Fig. 4. Jugular veins and lateral masses of l>la.^tema in the .sheep, 

 soon after the latter have joined across the middle line. 

 a. The triangular absorjjtion of the cervical portion, which is 

 the first indication of the sepai-atiou of the thyroid. 

 Fig. 5. The next stage, in which the thyroid is more distinct. 

 Fig. 6. The thyroid is now quite distinct, and ditl'ers from the thj-nius 

 in being opaque ; the latter exhil.dts opaque spots in a semi- 

 transparent matrix. 

 Fig. 7. The thyroid and thymus have assumed their perfect form. 

 Fig. 8. A portion of the supra-renal capsule of the adult gi-een monkey, 

 slightly compressed. It exhibits the nunute nucleated par- 

 ticles of which it consists. Among these, at pretty regular 

 distances, are seen the germinal spots. 

 Fig. 9. A portion of the thymus of the brown bear, slightly compressed. 

 It exhil)its the nucleated particles of which it consists. These 

 are grouped in spherical masses around centres fi-om which 

 they appear to have derived their origin. 

 Fig. 10. A portion of the thymus from a human foetus. It has been 

 taken fmm the surface of the gland, so as to exhildt the 

 areolar hbies which form its delicate capsule. The pressure 

 of the glas.s plates has almost obliterated the spherical 

 groujiiiig in the cells. 

 Fig. 11. A portion of the membrane which covered the contiguous sur- 

 faces of the lobes of the thymus of a human fcctus (the 

 membrane lining the re.sei-voirs of Sir A. Cooper). It has 

 the s;ime .structure as in Fig. 10. It exhibits no germinal 

 membrane, but consists of an areolar or fibrous textun- inter- 

 mixed with the cells of the organ, the fibres being more 



