104 PROCEEDINGS OF THE AMERICAN ACADEMY 



I believe, that what KoHiker ('79, p. 596) has ventured to suggest as 

 possible is iu reality the ouly rational explanation of the presence and 

 position of these nuclei. They exist, as Kolliker has said, indepen- 

 dently of the blood-vessels. It is also quite certain, from the position 

 they occupy when first seen, that they must have been derived from 

 the gray matter of the coid. 



The columns have by this time (PI. III. fig. 6) become thickened 

 along their median edges, and begun to grow ventralwards along the 

 outer margins of the tract occupied by the horn fibres. 



The posterior columns are already divided into numerous fasciculi of 

 various shapes by connective-tissue septula, which have in general a 

 radial direction extending from the gray substance toward the pia 

 mater. 



When embryos have reached the length of 85 mm. the Burdach'sche 

 Keilstriiuge (Pi. III. fig. 11, B) extend in the cervical region about 

 half-way from the periphery to the central canal, and have pressed 

 together closely the distal ends of the above-described cords, which 

 now may properly be called the Goll'sche Keilstriinge. The latter are 

 separated from each other only by the partition of horn fibres before 

 mentioned. 



At their distal edges Goll's Keilstriinge are distinct from Burdach's, 

 being separated from them by processes of connective tissue which are 

 continuous with the pia mater ; at their proximal margins, however, 

 they can only be distinguished from them by the different directions 

 which the fasciculi take in the two cases ; in the former, their ventral 

 edges trend towards the median plane, whereas the corresponding edges 

 of the latter have a direction away from this plane. They cannot be 

 distinguished by their histological structure ; neither by the size of the 

 nerve fibres nor by the size and arrangement of the fasciculi. Both 

 are composed of fine longitudinal fibres arranged in fasciculi which are 

 separated from each other by connective tissue. 



The nuclei have nearly all disappeared from the Goll'sche Keil- 

 strange, the few that remain presenting a more ragged appearance than 

 in the previous stages. 



Tlie further development of this region may be summarized in a few 

 words. It results from the apparent infolding of the posterior columns 

 that the white substance approaches nearer and nearer to the canal, 

 pushing before it the transverse connective-tissue fibres, which thus 

 come to make a broad ventrally convex curve, the ends of which 

 (PI. III. fig. 11) sweep up along the lateral margins of Burdach's 

 Keilstrange. 



