590 LOCY. 



EXPLANATION OF PLATE XXVIII. 



A series of partial dissections of embryos of Squalus in such a way that the 

 brain-walls have been laid bare. Figs. 46-49 X 45 diameters. Figs. 50-56 X 10 

 diameters. 



Fig. 38. Embryo with open neural groove. The epidermal layer and the 

 mesoderm have been removed from the sides of the brain-wall ; behind the optic 

 vesicle is seen a bilobed protuberance — the combined mid-brain vesicle and the 

 anterior accessory optic vesicle. 



Fig. 39. Older embryo with neural canal partly formed. The specimen shows 

 the same condition of mid-brain vesicle and that of the anterior accessory optic 

 vesicle. 



Fig. 40. The brain-walls of an embryo of the same age as that shown in Fig. 32. 



Fig. 41. Embryo before dissection, showing especially well the contours of the 

 brain-walls. 



Fig. 42. The same embryo after exposure of the brain-walls by dissection. 

 The thalamencephalon is well exhibited. The mid-brain is bilobed. 



Fig. 43. Sketch of partially dissected embryo just after the appearance of the 

 auditory vesicle. 



Fig. 44. The exposed brain-walls of an embryo slightly older than that repre- 

 sented in Fig. 34. 



Fig. 45. Brain of embryo about same age as that in Fig. 35. The auditory 

 vesicle has been left in position. The mid-brain is now indistinctly trilobed. 



Fig. 46. Brain-walls of embryo with the optic vesicle removed. About the 

 same age as the preceding. 



Fig. 47. Dissection of the brain of the embryo of which Fig. 36 is an external 

 view. The mid-brain is distinctly trilobed. There are eight clearly marked seg- 

 ments in the hind-brain. 



Fig. 48. Dissection of embryo slightly older than the one represented in 

 Fig. 37. The thalamencephalon is now definitely marked out by furrows ; it 

 bears upon its summit two rounded confluent protuberances. 



Fig. 49. Dissection of brain of embryo somewhat older than the preceding. 

 The thalamencephalon is clearly defined. The posterior protuberance from its 

 roof has grown much faster than the anterior one. The former is the beginning 

 of epiphysis. There are nine neural segments in the hind-brain. 



The embryos are now too large to represent advantageously on the same scale, 

 and in the following figures the scale of magnification is reduced from 45 to" 10 

 diameters. 



Fig. 50. Shows embryo of same age as that represented in Fig. 32. 



Fig. 51. Embryo of nearly the same age as that represented in Fig. 34. 



Fig. 52. Partly dissected embryo of about the same age as that represented in 

 Fig. 36 and again in Fig. 47. 



Fig. 53. Brain-walls of an embryo just older than that shown in Fig. 49. 



Figs. 54 and 55. Successively older embryos to show especially the changes in 

 the thalamencephalon and the outgrowth from its roof of the epiphysis. 



Fig. 56. The same brain shown in Fig. 55, with the cerebral lobes removed 

 and turned so as to view directly against the epiphysis. 



