478 Lela;id Griggs. 



brates since they are not seen in any adult vertebrates even in 

 the simplest forms. The nucleus of the nervous system is a 

 segmented solid axis but the complexity of this axis is consider- 

 able when we take into consideration the peculiar history of the 

 first neuromere. And again there is evidence of the grouping 

 of neuromeres into tagmata, a process which must have had an 

 ancient origin in the phylogeny of vertebrates. The complexity 

 becomes greatly increased when it is considered that to this neural 

 axis are added accessory structures such as germinal grooves, 

 retinal spots, optic lobes and neural crest, the absorption of which 

 into the neural axis takes place very early in embryonic life. It 

 may readily be admitted that several more accessory structures 

 are really represented although they cannot be detected in Ambly- 

 stoma. At any rate enough has been proved to show that the 

 very early anlage of the nervous system of so simple a vertebrate 

 as Amblystoma is comparatively complex. 



Second. ''The number and nature of the primitive neural 

 segments." The exact number of segments in the brain is 

 perhaps of less importance than the grouping of segments into 

 tagmata. It has been shown that the anlage of the fore-brain, 

 the mid-brain and the cerebellar region is a tagma of four neuro- 

 meres. It is possible that the exact number of neuromeres may 

 vary in different vertebrates, the tagma theniiselves forming the 

 invariable units. These early segments are evidently true 

 primitive neuromeres while the later divisions of the neural tube 

 are evidently secondary due to the presence of secondary struc- 

 tures such as the optic lobes. 



Third. ''The line of modification" of neuromeres. It has been 

 shown that the vertebrate brain is not derived solely from a 

 simple series of neuromeres but that its anlage is complex contain- 

 ing structures lying lateral to the segmented neural axis. Thus 

 the process is not merely a "modification of neuromeres" but com- 

 prises a fusion of more or less independent anlagen. To the first 

 neuromere after it has been folded down into the infundifular 

 depression are added the eye vesicles, the origin of which is lateral 

 to the neural axis. Possibly also the optic thalami and the 

 parietal eye which appear later in the walls of this region have a 



