84 CARNEGIE INSTITUTION OF WASHINGTON. 



were used so that the changes could be followed, death of the cell 

 resulting in about half an hour. In the nucleus the chromatin material 

 was contracted into a dense, deeply staining mass and the nuclear sap 

 was expelled in the form of clear fluid vacuoles, the process resembling 

 the segregation of chromatin material and nuclear sap in mitosis. In 

 both potassium experiments and mitosis the mitochondria became 

 broken up into rods and granules and transformed into vesicles. In 

 both, also, there was condensation of the cytoplasm. It was found that 

 the centrosphere was not affected; this, together with the fact that the 

 centrosphere enlarges in cell degeneration, may be taken as evidence 

 that this structure is not the dynamic center of the cell, as has been 

 previously maintained, but is rather to be looked upon as a degenerat- 

 ing area. 



CENTRAL NERVOUS SYSTEM AND SPECIAL SENSE-ORGANS. 

 The Fetal Brain. 



During the winter of 1920, spent in this laboratory as a guest, Pro- 

 fessor George B. Jenkins utilized our embrj^ological collection and 

 records for determining the relative weight and volume of the compo- 

 nent parts of the brain of the human embryo as found at different 

 stages of development. Dr. Jenkins's work has now been completed 

 and published. To represent the first half of intrauterine life he 

 selected 8 specimens (the youngest being 4.3 mm. long) which had been 

 prepared in serial sections. Models of the brains were made with wax 

 plates and these were subdivided into 11 component parts and the 

 weight and volume and the subdivisions determined. During the 

 second half of pregnancy the relative volume of the different parts 

 remains more constant, so that 2 specimens were sufficient to cover 

 this period and the brains were large enough to dissect into parts 

 corresponding to the younger specimens. The weight and volume of 

 each part could therefore be determined directly without resort to 

 serial sections and modeling. In this manner Dr. Jenkins was able to 

 trace the original predominance of the primary centers and their 

 gradual loss in relative volume coincident with the growth of the 

 secondary centers. It is of interest to note that in a 4-mm. embryo 

 the cerebrum constitutes only 7 per cent of the volume of the brain, 

 whereas in the later fetal stages it forms nearly 90 per cent. In the 

 younger stages about one-half of the brain volume is accounted for 

 by the nuclei of origin of the cranial nerves and their associated fibers. 



Rudimentary Head Cavity. 



In studying some of our younger human embryos. Dr. Joseph L. 

 Shellshear, of the University College, London, succeeded in identifying, 

 in the neighborhood of the glossopharyngeal ganglion, a cavity of 

 the axial mesoderm which appears to be homologous with the head 

 cavities that give origin to the muscles of the eye. Arising from the 



