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



and ventrally, the developing mammillary bodies. In older subjects, therefore, 

 this anteromesial incision must, on each side, extend between the mesencephalon 

 and the thalami, to end just caudal to the epiphyseal attachment dorsally and the 

 mammillary bodies ventrally. 



Having divided the entire encephalon into the three primary vesicles, the suc- 

 ceeding steps consist in separating these structures into their more prominent 

 constituent parts. In the 4 mm. embryo it was not possible to accurately remove 

 the cerebellar rudiment. In the 16 mm. embryo, in the case of the hindbrain, the 

 cerebellum could be removed from the stem. In the early stages this was accom- 

 plished by removing the prominent anterior rhombic lip on either side, which corre- 

 sponds to the cerebellar rudiment, as shown in figure 2. This mass, at first a promi- 

 nent roll or lip, becomes more and more sharply defined in the succeeding stages and 

 is delimited from the mesencephalon by the transverse groove mentioned above. 

 Owing to the difficulty of accurately separating the pons and the medulla, they were 

 considered as a unit mass. The mesencephalon was considered as a single mass 

 throughout the series. The fore-brain was then subdivided into the telencephalon 

 and the diencephalon. In the early stage this division was accomplished by cutting 

 along the thalamic margin, which extends as a slight groove along the dorsal border 

 of the optic evagination and its later developing stalk connected with the dien- 

 cephalon. In the older specimens the separation was effected by cutting through 

 the internal capsule between the thalamus and the corpus striatum dorsally, the 

 stria terminalis being used as a guide; ventrally the incision was made along the 

 lateral margin of the optic tract, leaving this structure connected with the thalam- 

 encephalon. Thus the diencephalon comprises the optic tracts, the thalamus, 

 the epithalamus, and the hypothalamus (except the hypophysis, which could not be 

 considered in all cases). The diencephalon was not subdivided. The telenceph- 

 alon in all stages was subdivided into the neopallium and the archipallium. In 

 the early stages the line of incision was made to include the shallow depression in 

 the lumen of the ventral wall of the first vesicle, lateral to the lamina terminalis 

 and anterior to the optic stalk. In the older specimens (16 mm. and upwards) this 

 rudiment is larger and consequently more easily separated from the remainder of 

 the vesicle. Here there is a considerable ventral prominence appearing lateral to 

 the lamina terminalis anterior to the torus opticus. This comprises the olfactory 

 bulb and tract. The latter extends along the lower or ventral margin of the 

 depressed cortical area over the base of the corpus striatum. In older subjects 

 (50 mm. and upwards) the anterior commissure, paraterminal body, fornix, and 

 hippocampus could easily be made out by microscopic study and as they became 

 differentiated they were removed and studied. 



The method employed in isolating these various structures in the older modeled 

 embryos was essentially the same as that followed in dissecting the brain of the 

 term fetus, which was as follows : The cord was separated from the medulla at the 

 conventional level (in these specimens the bodies had been injected with a 10 per 

 cent solution of formalin and the brain, after removal, had been kept in the same 

 solution until studied) and the upper part of each cerebral hemisphere removed 



