DIENCEPHALON 233 



differentiation is much further advanced, though there is no radical 

 change in the topographic arrangement of the areas involved ('38, 

 figs. 15-18). But, by the time the larvae begin to feed (Harrison's 

 stage 46), the parts have shifted their positions toward the adult con- 

 dition ('386, figs. 1, 2), which is fully attained in midlarval stages (38 

 mm, in length) of Amblystoma tigrinum ('39a, fig. 1). 



These changes are brought about in two ways. There is, in the first 

 place, a gradual straightening of the two great flexures of the neural 

 tube, with a resulting rearrangement of the several areas in relation 

 to the long axis of the brain and to one another. In the second place 

 and accompanying these changes in the general shape of the fore- 

 brain, the regions of most rapid proliferation and difi'erentiation of 

 cells shift their positions. There results a change in the pattern of the 

 ventricular eminences and the intervening sulci. The original trans- 

 verse ridges and sulci are broken up, and their parts are rearranged in 

 the longitudinal series which we see in the adult brain. 



The details of these changes have been described during recent 

 years by a number of workers in Professor Holmgren's Zootomical 

 Institute at Stockholm. Rudebeck ('45), in his admirable paper on 

 the development of the forebrain of lungfishes, shows that the se- 

 quence of these changes is essentially the same in Dipnoi, Urodela, 

 and Anura. In the youngest stages, proliferation of cells is most active 

 along the lines of the primitive sulci. These active zones enlarge, and 

 this results in obliteration of the primary sulci or a shift in their posi- 

 tions. The primary grooves, accordingly, do not mark the boundaries 

 of the primordia of the different diencephalic centers, but, on the 

 contrary, they are zones of more active growth from which these cen- 

 ters are developed. In some regions the original proliferation grooves 

 disappear, in others they are gradually displaced and so become limit- 

 ing sulci at the boundaries of the differentiated areas of the adult. 



My interpretation of these changes as outlined above differs some- 

 what from that of Rudebek as summarized on page 63 of his mono- 

 graph. Certainly, the details of these developmental processes vary 

 from species to species, and examination of still more species of 

 vertebrates along the lines of the critical studies made in the Stock- 

 holm Institute will doubtless clarify the questions which are still in 

 controversy. One is impressed by the close resemblance of this devel- 

 opment in lungfishes and amphibians, which suggests that these 

 groups of animals are more intimately related phylogenetically than 

 has hitherto been generally admitted. 



