EXPLANATION OF THE PLATES. xiii 



around the trunks of the omphalo-mesenteric vessels ; this is 

 the posterior part of the lateral mass of blastema marked d 

 in Fig. 1, and becomes in the course of development the 

 supra-renal capsule. 

 Fig. 3. An early embryo of the sheep. 



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



6. Heart. 



c. Ductus Cuvieri entering the auricle, and receiving 



d. The jugular, and 



e. The cardinal vein. 



/ The lateral blastema. 



g. Wolffian body. 



h. Umbilical cord, to which is passing 



i. The allantois. 



j. The omphalo-mesenteric artery, and 



k. Omphalo-mesenteric vein ; traces of the umbilical vessels are 



also seen in the parietes of the abdomen. 

 I. The liver and intestinal tube. 

 m. Lungs. 

 Fig. 4. Jugular veins and lateral masses of blastema in the sheep, 



soon after the latter have joined across the middle line. 

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



the first indication of the separation of the thyroid. 

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

 Fig. 6. The thyroid is now quite distinct, and differs from the thymus 

 in being opaque ; the latter exhibits 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 green monkey, 

 slightly compressed. It exhibits the minute 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 exhibits the nucleated particles of which it consists. These 

 are grouped in spherical masses around centres from which 

 they appear to have derived their origin. 



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

 taken from the surface of the gland, so as to exhibit the 

 areolar fibres which form its delicate capsule. The pressure 

 of the glass plates has almost obliterated the spherical 

 grouping 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 foetus (the 

 membrane lining the reservoirs of Sir A. Cooper). It has 

 the same structure as in Fig. 10. It exhibits no germinal 

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

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



