SEPTUM, HIPPOCAMPUS, PALLIAL COMMISSURES 405 



corpus callosum in pig embryos and must agree with Goldstein 

 and Werkman. The intermediate view of Marchand, that there 

 is a very early fusion of a small area of the hemisphere walls 

 which is thereafter stretched by the growth of the commissures 

 may be explained by two factors. First is the thickening of the 

 lamina terminalis (and of the lamina supraneuroporica as defined in 

 the present paper) by the proliferation of neuroglia as described 

 in detail by Werkman. This results in a thickened ''Glia- 

 shicht" through which the fibers may run. Second, there is a 

 migration of cells (neuroblasts) into the lamina supraneuroporica 

 from the hippocampal primordia at either side just as there is 

 a secondary thickening of the lamina terminalis by migration of 

 cells from the paraterminal body to form a bed for the anterior 

 commissure. In figures 7 and 24 is seen a ridge extending upward 

 and forward from the lamina supraneuroporica in the hemisphere 

 wall. This ridge indicates the line along which pallial (hippo- 

 campal) commissure fibers enter the lamina and cells migrate 

 into this lamina to form the commissure bed described above 

 in several mammals. This thickening of the roof plate at first 

 by glia and afterward by neuroblasts produces the massive 

 •lamina supraneuroporica which is characteristic of vertebrates 

 and may explain Marchand's conception of an early area of 

 fusion and Gronberg's concrescentia primitiva. Space can not 

 be taken for a full account of the development of the commis- 

 sure. The reader is referred to the description of figures 78 to 

 81. 



That there is no fusion of the medial walls and that the com- 

 missure bed is merely expanded by intussusception of callosal 

 fibers is evidenced by the appearance of the commissures in the 

 lamina supraneuroporica (upper border of lamina terminalis of 

 authors) (Marchand, figs. 7, 9; Werkman, figs. 5, 17, 24); the 

 fact that the corpus callosum in all stages of growth has a smooth 

 and regular border upon which the falx and anterior cerebral 

 artery lie as if they had been pushed before the developing com- 

 missure (Goldstein, p. 46 and fig. 15); and the fact that the 

 commissures are always completely surrounded by a thicker or 

 thinner layer of nerve cells (Blumenau); which I can confirm 



