592 



LOCY. 



EXPLANATION OF PLATE XXIX. 



Figs. 57-62 a continuation of the series of dissections shown on the two pre- 

 vious plates. All X 10 diameters. Figs. 65-87 X 45 diameters. 



Fig. 57. Side view of brain of an older embryo than that sketched in the fore- 

 going figure. The neural segments have now become obliterated, and the three 

 lobes of the mid-brain (secondary divisions) are no longer distinguishable. The 

 epiphysis is well developed ; in front of it is a semicircular fold of the brain-wall; 

 this structure is the remnant of the elevation which started in front of the epiph- 

 ysis on the roof of the thalamencephalon (see Figs. 48 and 49). It has become 

 reduced by compression between the rapidly growing adjacent brain regions. The 

 paraphysis is also visible in this drawing. 



Fig. 58. View of the same brain from above looking into the cavity of the 

 fourth ventricle. 



Fig. 59. The same brain after removal of the cerebral lobes and arranged so 

 as to show the epiphysis in front view. 



Fig. 60. Brain of older embryo showing substantially the same features as the 

 preceding. 



Fig. 61. The cerebral lobes of the same brain after removal. Note the 

 paraphysis arising from the posterior part of the roof of the prosencephalon. 



Fig. 62. The same brain after removal of prosencephalon and arranged so as 

 to show to best advantage the epiphysis with its stalk. 



Fig. 63. Horizontal section of embryo shown in Fig. 25 to show the general 

 appearance of the metameres in section. X 50. 



Fig. 64. Horizontal section of embryo sketched in Fig. 26 showing metameres 

 in section, and adjacent layer of mesoderm. X 50. 



Fig. 65. Section of head of Torpedo ocellata through the optic vesicles at the 

 time of their first appearance. Compare this with Zieglers' PI. IV, Fig. 19*. 



Figs. 66, 67, 68, 69, 70. Successive sagittal sections of embryo just after 

 closure of the neural groove and prior to the appearance of the ear vesicle. Show 

 the primary fore-brain, the mid- and hind-brains, and three very prominent neural 

 segments of the hind-brain. The segments are the seventh, eighth, and ninth 

 respectively. When the ear is first differentiated it arises opposite the ninth 

 segment. 



Fig. 71. Deeper section of the same embryo showing a curved line of meso- 

 derm over the mandibular cavity. 



Fig. 72. Sagittal section of embryo near the median plane after the formation 

 of the auditory saucer and the complete closure of the neural groove. The five 

 neural segments of the fore- and mid-brain are exhibited. The second neural 

 segment nearly coincides in position with the neuropore. 



Figs. 73, 74, 75. Three successive sagittal sections of an embryo of about the 

 age of that drawn in Fig. 34. It is slightly older, shows well the neuromeres of 

 the hind-brain. The ear capsule is in the space of the tenth neuromere. In front 

 of it in Figs. 73 and 74 are seen the roots of the seventh and eighth nerves. 



Figs. 76-87. Sagittal sections of the specimen photographed as Fig. 17, PI. 

 XXVI. Giving evidence of a series of cup-like depressions on the neural plate of 

 head and trunk. The series of these cupped areas is terminated in front by the 

 optic vesicles. On the cephalic plate they are relatively large and resemble the 

 primary optic vesicles in mode of origin and in structure. How far the series 

 extends into the trunk I have not been able to determine. 



