RELATIVE WEIGHT AND VOLUME OF COMPONENT PARTS OF FETAL BRAIN. 53 



to 10.19 per cent, of which the medulla-pons 

 represents 6.23 per cent and the cerebellum 3.96 

 per cent, thus showing the continued and rapid 

 increase of the prosencephalon, especially the 

 neopallium, over the other parts. 



No. 1400-22, 80 mm., estimated age 13J^ weeks. 



A still more decided advance in bulk and dif- 

 ferentiation is here observed (fig. 6). The 

 cerebral hemispheres are considerably more 



ebel. 



Flo. 6. Left view of model of brain-stem, embryo No. 

 1400-22, crown-rump length SO mm. X 6. 



extensive, the telencephalon totaling 80.56 per 

 cent of the entire weight of the encephalon. Of 

 this the neopallium constitutes 65.55 per cent 

 and the corpus striatum 11.23 per cent, a total 

 of 76.78 per cent. The archipallium weighs 3.78 



creased to 88.6 per cent. The mesencephalon 

 presents a sligh decrease, weighing now 4.78 

 per cent. The total rhombencephalon has also 

 lost relatively, weighing 6.62 per cent, of which 

 the cerebellum comprises 2.89 per cent, the 

 medulla-pons 3.73 per cent. 



No. 1400-25, 119 mm., estimated age 16 weeks. 



The advance in general bulk is distributed in 

 this embryo (figs. 7 and 8) as follows: The total 

 prosencephalon constitutes 91.91 per cent of the 

 brain weight, of which the diencephalon com- 

 prises 6.29 per cent a decrease, the telenceph- 

 alon having meanwhile increased to 85.62 per 

 cent. To this the neopallium contributes 69.55 

 per cent, the corpus striatum 11.64 per cent, and 

 the archipallium 4.43 per cent. Of the latter the 

 olfactory bulb shows a decreased weight of 0.28 

 per cent, and the paraterminal body, fornix, and 

 hippocampus a weight of 4.15 per cent a slight 

 rise. The mesencephalon has fallen to 2.06 per 

 cent. The total rhombencephalon weighs 6.03 

 per cent, of which the cerebellum forms 2.74 per 

 cent and the medulla-pons 3.29 per cent, all of 

 which weights are practically identical with those 

 of the corresponding parts in the 80 mm. speci- 

 men, so that the telencephalic gain is balanced by 

 the losses sustained by the diencephalon and the 

 mesencephalon, the fractional increase in the 

 rhinencephalon being likewise accounted for. 



Cerebel 



JiMIil 



\ 



Medulla 



Hypothal. Paraterm. body 



*'->.-. 



Paraterm.body 



Olfact.bulb 



8 



HippocQmp.8t fornix 



FIG. 7. Mesial view of model of brain-stem of embryo No. 1400-25, crown-rump length 119 mm. X 2.5. 

 Flo. 8. Mesial view of right half of telencephalon of embryo shown in figure 7. X 2.5. 



per cent, the olfactory bulb contributing 0.35 per 

 cent and the paraterminal body, fornix, and 

 hippocampus 3.43 per cent; all of which agree 

 substantially with the figures given for the 

 corresponding parts of the preceding embryo. 

 On the other hand, the neopallium has increased 

 and the diencephalon has diminished to 8.04 

 per cent, the total prosencephalon having in- 



No. 1400-28, 156 mm., estimated age 19 weeks. 

 This specimen (figs. 9, 10, 11, and 12) was 

 studied and published by Dockeray (1915), and 

 his findings have been rearranged and incor- 

 porated into this study. The total prosenceph- 

 alon has attained the enormous weight-value 

 of 93.69 per cent of the total brain weight, the 

 diencephalon dropping to 4.81 per cent and the 



