. Contributions to embryology. Embryology. 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 per cent of the total, as compared with twice that amount in tlie 8 weeks' embryo. The total prosencephalon has nearly doubled its relative weight, being now per cent, the diencejihalon drojiping to per cent, and the telencephalon increasing to per cent, of which the archipallium constitutes per cent and the neopallium per cent, well back over
. Contributions to embryology. Embryology. 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 per cent of the total, as compared with twice that amount in tlie 8 weeks' embryo. The total prosencephalon has nearly doubled its relative weight, being now per cent, the diencejihalon drojiping to per cent, and the telencephalon increasing to per cent, of which the archipallium constitutes per cent and the neopallium 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, mm., estimated age 12^ weeks This specimen still more closely approaches the full adult differentiation. The cerebral hemi- spheres have grown in all directions to a con- n e o p Q If , ,, â '';^^^ni:\ / ^^ ^^_, Archipal, «_^ v_ - [Fornix &Hippocomp ' ^ ^5^-v^n-?;l .^''' Q' - Mese'nceph. â¢4â .i' L. Hypothal Fig. 3.âMesial view of right hemisphere of embryo No. 96, crown-rump length 60 mm., showing sub- divisions of archipallium. X 5. FiQ. 4.âLeft view of model of brain-stem (same embryo as shown in fig. 3), showing its subdivisions. X 5- Fig. 5.âMesial 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 per cent of the total brain. The various parts of the archi- pallium could also be isolated, giving an olfac- tory bulb weighing per cent, and the para- terminal body, fornLx, and hippocampus, which together weighed per cent of the total. The pontine flexure is much less acute and the cere- bellum, as yet rudimentary, is a narrow, beak- like struc
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