52 RELATIVE WEIGHT AND VOLUME OF COMPONENT PARTS OF FETAL BRAIN. 



siderable decrease in all three values. The 

 mesencephalon has likewise decreased, weighing 

 in this specimen but 7.09 per cent of the total, as 

 compared with twice that amount in the 8 weeks' 

 embryo. The total prosencephalon has nearly 

 doubled its relative weight, being now 80.14 per 

 cent, the diencephalon dropping to 12.77 per 

 cent, and the telencephalon increasing to 67.37 

 per cent, of which the archipallium constitutes 

 3.52 per cent and the neopallium 51.18 per cent, 



well back over the diencephalon, encroaching 

 upon the mesencephalon. A curious feature, 

 noted only at this age, is that the corpora striata, 

 the diencephalon, and the rhombencephalon, are 

 all of equal weight value. 



No. 1400-20, 69.5 mm., estimated age 12J^ weeks 



This specimen still more closely approaches the 

 full adult differentiation. The cerebral hemi- 

 spheres have grown in all directions to a con- 



f f' : $; ;' vyv^s 



lU iH 



lOlfad.bulb / 

 Fornix G H ippocamp ''<;.'..;.,-. 



V 



Corpus s/ 



'u~. .y^-.* '*''.'.'.??>' '. ". '' f f '~',' '"' 



FIQ. 3. Mesial view of right hemisphere of embryo No. 96, crown-rump length 50 mm., showing sub- 

 divisions of archipallium. X 5. 



FIG. 4. Left view of model of brain-stem (same embryo as shown in fig. 3), showing its subdivisions. 

 X 5. 



FIG. 6. Mesial view of hemisection of model shown in fig. 4, showing subdivisions of diencephalon. 

 X 5. 



a tremendous increase over all preceding stages. 

 The corpora striata, which could be isolated in 

 this specimen, weighed 12.67 per cent of the 

 total brain. The various parts of the archi- 

 pallium could also be isolated, giving an olfac- 

 tory bulb weighing 0.3 per cent, and the para- 

 terminal body, fornix, and hippocampus, which 

 together weighed 3.22 per cent of the total. The 

 pontine flexure is much less acute and the cere- 

 bellum, as yet rudimentary, is a narrow, beak- 

 like structure (fig. 3). The mesencephalon 

 shows a distinct caudal projection overhanging 

 the median part of the roof of the cerebellar 

 rudiment. The cerebral hemispheres project 



siderable degree, the total prosencephalon weigh- 

 ing 84.84 per cent of the whole. Of this the 

 diencephalon contributes 11.47 per cent, a 

 steady relative decrease, despite the increase in 

 bulk of the developing thalami. The archi- 

 pallium has relatively decreased, weighing but 

 2.87 per cent, of which the olfactory bulb con- 

 tributes but 0.15 per cent and the paraterminal 

 body, fornix, and hippocampus 2.72 per cent. 

 The neopallium, on the contrary, weighs 60.62 

 per cent and the corpus striatum 8.88 per cent, 

 making a total of 69 per cent of the entire 

 encephalon. The mesencephalon has dropped 

 to 5.97 per cent and the total rhombencephalon 



