212 CLARKE. 



EXPLANATION OF PLATE XII. 



(Figures 32, 34, and 38 are from life, and were made by Mr. C. C Hayes. The others are from 

 preserved specimens, and are my own.) 



Fig. 29. From the ventral surface. 



Fig. 30. The same embryo from the dorsal side. 



Fig. 31. A ventral view. 



Fig. 32. A lateral and ventral view from a living specimen. The embryo is now 

 partly turned upon its side. 



Fig. 33. A little later than 32. The pharyngeal clefts are forming; also the eye 

 and ear. 



Fig. 34. This is from life, and shows especially well the neuromeres. The first 

 and second pharyngeal clefts are developing. 



Fig. 35. This shows the connection of the fourth with the third cleft; the olfactory 

 pit is forming; several neuromeres still persist in the hind-brain; the heart is folding 

 upon itself. 



Fig. 36. The formation of the mouth region is seen here and the pharyngeal 

 arches and clefts. 



Fig. 37. The embryo is turned partly on its side, showing the nasal groove reach- 

 ing to the mouth, and the outlines of the mouth; also the rudiment of the anterior 

 limb. 



Fig. 38. From life. This is not so advanced an embryo as that represented by 

 Fig. 37, the limbs having not yet appeared. The first cleft is seen to be bifurcated 

 dorsally; the rudiment of the fifth nerve has appeared; the allantois has become 

 prominent; also the arteries and veins of the yolk-sac. 



Fig. 39. Another embryo, showing also the bifurcated first cleft. 



Fig. 40. The mode of formation of the first and second pharyngeal clefts is here 

 shown. The drawing is made from a mounted specimen by transmitted light. 



Fig. 41. Here the first three clefts are formed, and the fourth and fifth are con- 

 nected with the third. 



Fig. 42. A latero-longitudinal section, showing the five pharyngeal clefts. 



