in the Fcetus of Vertebrated Animals. Ill 



21. (Prom Baer) Section of the chick in ovo on the fourth daj. 



22. (From Rathke) Embryo of the chick on the fifth day. Magnified 



one diameter. 



23. (From do.) Longitudinal and vertical section of the head and neck 



of the chick on the fifth day, shewing the interior of the pharynx 

 and the remains of the branchial apertures. 



24. (From do.) Heart and anterior part of the neck of the same embryo, 



sliewing the infer, maxilla and operculum. 



25. Posterior view of the heart and interior of pharynx, at a correspond- 



ing period, shewing the branchial arches given ofi" by the bulb of 

 the aorta. A, Section of the extremity of the bulb. 



26. (From Rathke) A, The lower, and B, The lateral view of the lungs 



and trachea of the chick on the fifth day, with the oesophagus. 



27. (From do.) The same on the sixth day. 



28. (From do.) The same on the seventh day. 



(From do.) The lungs, &c. on the eleventh day, shewing the trachea, 

 aorta, pulmonary arteries, communicating vessels, and the union of 

 the cellular, with the bronchial parts of the lungs. 



30. (From Burdach, but reversed.) Diagram shewing the branchial di- 



visions of the aorta of the chick on the lower side of the pharynx, 

 and the mode of their transformation into the aortic and pulmon- 

 ary vessels. 



31. The position of the ductus arteriosi relatively to the oesophagus in 



the chick about the 12th day. 



Mammalia. 



32. (From Bojanus) Sketch shewing the relative position, size, &c of 



the allantois and yolk-sac, &c. in the fcetus of the sheep about three 

 weeks old. A, The foetus and umbilical vesicle magnified. B, A 

 imall part of the fcetal placenta or cotyledon of the cal^ shewing, 

 the processes of the chorion on which the umbilical vessels are ra- 

 mified. 



33. (From Bojanus) The same in the dog of i4 days. 



34. The same in the rabbit of about fourteen days, with the placenta, 



A, The entire ovum. B, The chorion and umbilical vesicle opened,, 

 so as to shew the allantois and placenta. 



35. The head and neck of the embryo of the dog of three weeks, repre- 



sented by Baer, and copied from Fig. 8. of former Essay. 



36. The human embryo of about six weeks, in which I found two bran- 



chial apertures at least on each side of the neck, the heart exposed. 

 A, The natural size. 



37. (From Rathke) Anterior view of the neck of the foetus of the pig, 



represented in Fig. 9. of former Essay, shewing the branchial aper- 

 tures and operculum. 



38. Heart and branchial arches in the rabbit. Fig. 34. 



39. Diagram of the branchial arches of mammalia, and their transforma- 



tions, corresponding with that of birds by Burdach. A, Ductus- 

 arteriosus of mammalia when just formed. 



40. Posterior view of the heart and commencing lungs and trachea of 



the rabbit, Fig. 34. Magnified three diameters. 



41. (From Rathke) The tongue, trachea, and lungs of the foetal horse 



seen from above, twice the natural size* A, The same seen from 

 below. B, Section. 



42. (From do.) Lungs of the pig farther advanced, twice the natural 



size, seen from below. A, The same, seen from above. B, Section. 



43. The heart, lungs, pulmonary and aortic vessels, and ductus arterio- 



sus of the human embryo of ten weeks, twice the natural size. 



