482 J. B. JOHNSTON 



Tho luatoriiil in botli cases was boMutifully sectioned and stained. 

 The brains of freshwater turtles are so similar that the younger 

 stages of Chelydra can be readily compared with the later stages 

 of Chrysemys for the rather general purposes of this study. 



ARIOAS OF PROLTFP]RATION IN THP^ EARLY EMBRYO IN RELATION 

 TO CELL MASSES OF THE ADULT BRAIN 



Broadly speaking, three periods are to be recognized in the 

 development of cell masses in the telencephalon: a period of 

 evagination of the hemispheres and formation of indifferent 

 cells by mitosis of the germinal cells; a period of form changes 

 and further proliferation of cells in which the successive forma- 

 tion of functionally ind pendent cell masses is noticeable; and 

 a period during which the growth of fiber bundles and other 

 factors bring about the definitive form of the adult cell masses. 



In the first period the wall of the telencephalon consists of 

 a deep layer of spongioblasts and dividing germinal cells, a 

 thick layer of indifferent cells and a superficial clear layer or 

 marginal veil of very varying thickness. The thickest part 

 of the marginal veil forms the floor of the groove along the 

 lower border of the hemisphere, at its junction with the brain 

 stem. This is the course of the future lateral forebrain bundle 

 or crus cerebri. The indifferent cells form a continuous layer 

 with no indication of individual masses. 



The second period begins with the appearance of independent 

 cell masses separated by cell-free zones which show in sections 

 as clear lines or areas. Such cell-free zones in adult brains have 

 been commonly regarded as boundary lines between individual 

 cell masses which are presumed to have different functions; 

 i.e., centers or nuclei. Similar cell-free zones in the brains of 

 lower vertebrates have been used in recent years as landmarks 

 between important morphological regions, as the pallial and 

 basal areas. The writer has pointed out the need of caution 

 in interpreting these zones in this connection ('13 b, p. 382). 

 It is now to be noted that the formation of individual masses 

 of cells is dependent upon the proliferation of cells from differ- 

 ent regions of the germinal cell layer and upon the proliferation 



