RELATIVE WEIGHT AND VOLUME OF COMPONENT PARTS OF FETAL BRAIN. 51 



roof, showing a tendency to overlap the con- 

 stricted caudal extremity of the mesencephalon. 

 The cerebellum is not sufficiently differentiated 

 at this stage to permit of its delimitation. The 

 site of the pontine flexure is not indicated at this 

 stage, though both the cervical and the cephalic 

 flexures are well marked. 



No. 43, 16 mm., estimated menstrual age 7 weeks. 



This specimen (see fig. 2) presents a much 

 more advanced stage of development. The 

 tubular character is less pronounced and the 

 pontine flexure is well defined, serving to ac- 

 centuate both the cervical and cephalic flexures. 

 That portion of the rhombic lip constituting the 

 cerebellar rudiment was sufficiently well dif- 

 ferentiated to permit of its removal and study, 

 oh 



-r I ' 

 Q ; '-/ : 

 I' 



36.63 per cent of the total brain. Of this the 

 cerebellum comprises 9.74 per cent and the 

 medulla-pons 26.89 per cent, a slight increase for 

 the former, a considerable decrease for the latter. 

 The mesencephalon has increased to 14.76 per 

 cent, the total prosenccphalon to 48 per cent, to 

 which the diencephalon contributes 19 per cent 

 a slight shrinkage, and the telencephalon 28.84 

 per cent, a gain of 7 per cent over the preceding 

 stage. A further study of the telencephalon 

 shows an archipallium of 1.43 per cent, a tre- 

 mendous drop as compared with 18.09 per cent 

 at 16 mm. The neopallium has, on the other 

 hand, increased to 27.41 per cent, a complete 

 reversal of relative values. 



The basal ganglia, both the thalami and the 

 corpora striata, show a considerable increase in 



: 



' * : 



; ' 



FlQ. 2. Left profile view of reconstruction of lirain 

 of embryo No. 43, crown-rump length 

 16 mm. X 10. 



contributing 7.56 per cent of the total weight as 

 opposed to 38.11 per cent for the medulla-pons, 

 the combined weights giving a total of 45.67 per 

 cent for the rhombencephalon, a relative decrease 

 from the preceding specimen. The mesenceph- 

 alon has decreased to 12.52 per cent and the 

 diencephalon to 20.38 per cent, whereas the 

 telencephalon has increased to 21.43 per cent, 

 three times its weight in the younger embryo. 

 Of this the archipallium constitutes 18.09 per 

 cent and the neopallium 3.34 per cent, making a 

 total of 41.81 per cent for the prosencephalon, as 

 opposed to 31.3 per cent in the 4.3 mm. stage. 



No. 584<z, 25 mm., estimated menstrual age 8 weeks. 



This embryo presents a still further increase in 

 development, the rhombencephalon weighing 



Rhombeneep/, 



Archipal. 



FlO. 1. Left profile view of reconstruction of 

 the encephalon of embryo No. 148, 

 crown-rump length 4.3 mm., show- 

 ing manner in which the brain wag 

 subdivided. X 25. 



bulk at this stage. The walls of the vesicles also 

 show a corresponding increase in thickness, so it 

 will be observed that, although there has been a 

 marked increase in size and weight of the ence- 

 phalon up to this stage of development, it con- 

 sists in cell proliferation rather than in dedif- 

 ferentiation and does not permit of much added 

 study of individual parts. 



No. 96, 50 mm., estimated age 11 weeks. 



This embryo (figs. 3, 4, and 5) has advanced 

 much farther in development and permits of a 

 greater amount of subdivision of the model. 

 The total brain-weight is 514.58 grams, of which 

 the rhombencephalon comprises 12.77 per cent, 

 the cerebellum contributing 3.79 per cent and 

 the medulla-pons 8.96 per cent, a relatively con- 



