RELATIVE WEIGHT AND VOLUME OF COMPONENT PARTS OF FETAL BRAIN. 49 



by a transverse incision made just above the level of the fibers of the corpus callosum. 

 The occipital lobe on each side was then removed flush with the caudal margin of the 

 splenium and the basal surfaces of the temporal lobes were pared away until the 

 junction of the mesencephalon and diencephalon could be seen. The mesenceph- 

 alon was then separated from the rhombencephalon by an incision passing just 

 cranial to the transverse pontine fibers ventrally, and just caudal to the posterior 

 colliculi dorsally. The mesencephalon was next removed by an incision passing 

 anteromedially between the anterior colliculi and the pulvinars, just caudal to the 

 epiphyseal stalk dorsally and the corpora mamillaria ventrally. Next, the 

 olfactory bulbs and tracts were freed and removed by cutting each tract at its 

 junction with the base of the hemisphere. The two hemispheres were then 

 separated by cutting through the median sagittal plane of the corpus callosum, the 

 lamina terminalis, chiasma, and the basal structures, the incision passing between 

 the mammillary protuberances. The resulting blocks were treated alike; the 

 corpus callosum was lifted up and carefully dissected off from the fornix, exposing 

 that structure, as well as the thalamus, stria terminalis, and the corpus striatum. 

 The remainder of the cortex was removed anteriorly, with the head of the caudate 

 nucleus as a guide. Laterally, the cortex of the insula was pared down to the 

 lenticular nucleus and the temporal lobe removed, leaving the hippocampus, the 

 fimbrise, and the fornix intact. Following the fornix around anteriorly, its anterior 

 pillar and the mammillary and paraterminal bodies were removed together. The 

 thalamencephalon was then separated from the corpus striatum by cutting through 

 the internal capsule, following dorsally the line of the stria terminalis, and ventrally, 

 cutting just lateral to the optic tract, leaving that structure connected with the 

 diencephalon. The cerebellum was then separated from the remainder of the hind- 

 brain by cutting through the peduncles flush with the hemispheres. 



This method of subdivision gave 8 separate units for study: (1) the medulla- 

 pons, (2) the cerebellum, (3) the mesencephalon, (4), the diencephalon, (5) the 

 fornix and hippocampus, including the paraterminal and mammillary bodies, (6) the 

 olfactory bulb and tract, (7) the telencephalon, and (8) the corpus striatum. In 

 all of the specimens a regrouping of these units was made, giving us the following 

 additional units: (1) the total hindbrain, which comprises both the cerebellum and 

 the medulla-pons ; (2) the total telencephalon which was subdivided into archipal- 

 lium (including the olfactory apparatus) and the neopallium that is, the remainder 

 of the telencephalon, minus the corpus striatum, which in the more advanced 

 specimens was considered as a separate unit. In each instance the total weight and 

 volume of the encephalon were determined; then those of each of the subdivisions 

 were ascertained and the percentages calculated from the totals. The actual 

 weight and volume could both be determined in the case of the dissected specimens, 

 while for the modeled specimens the weight and volume of the entire model, or of a 

 part, were first determined. The weights of these wax models were simply carried 

 through, no attempt being made to ascertain the actual weight, since the relative 

 values would have been the same. The volume of the model was obtained by deter- 

 mining the volume of a gram of the wax used in making the model and multiplying 



