320 ANNALS Nr:W YORK Af'ArmMY OF sriKXf'Fi^ 



of tl)e cat — it is possible that in regions in which a tiaiiglionic crest is 

 not formed, honiodynamous elements remain incorporated and form 

 permanent constituents of the brain-wall. It ought not to be inferred, 

 from absence of a discrete ganglionic crest in any region, that its equiva- 

 lent is lacking, for it may simply have failed to separate from tlie 

 neuraxis. On the other hand, with regaj'd to the interpretation of the 

 forel)rain in terms of transverse segmentation, neuromeres, it would seem 

 that a complete dorso-vcnli'al segment of the neural tube should contain 

 ganglionic, alar and basal elements, and that failing any of these it is 

 something less and other than what the terra neuromere properly implies. 

 Now we have no evidence of the existence of the basal lamina in advance 

 of the nucleus of origin of the oculo-motorius, nor can we see the ad- 

 vantage of assuming its presence in front of the point at wliich evidence 

 of its existence ceases. Further, the transverse segmentation of the neu- 

 raxis is either the result of intrinsic factors, or what seems more probabl". 

 is at least in part secondary to the segmentation of the mesoderm. This 

 is myomeric in the trunk, branchiomeric in the head. That a secondary 

 segmentation of the neuraxis thus effected should result in a continuous 

 series of meristic equivalents seems on the face of it somewhat improb- 

 able. These, in brief, are the questions we have had chiefly in mind in 

 attempting an ontogenetic analysis of the forebrain of the eat. At the 

 same time it seemed desira1)le to record the general data concerning the 

 neuraxis in a close and fairly numerous series of young embryos, for as 

 yet the knowledge of these stages in mammals is far froiu extensive. 

 The list of embryos is as follows : 



Prior to the appearance of somites Nos. .S.^9. 400. 456. 550. 555 



One pair of mesodermic somites Nos. "t4. ."04 



Two pairs of mesodermic somites No. .I.'IO 



Tliree pairs of mesodermic somites No. ."O.'", 



Four pairs of mesodermic somites No. 40n 



Seven pairs of mesodermic somites Nos. .">SsT. ."S9 



Eight pairs of mesodermic somites Nos. .".'^.O. 5Sfi 



Nine pairs of mesodermic somites No. .5.^1 



Ten pairs of mesodermic somites Nos. 470. o.">2 



Twelve pairs of mesodermic somites Nos. ~tPA. ."47 



Tliirteen pairs of mesodermic somites No. *>!(> 



Fourteen pairs of mesodermic somites Nns. iss. .~4"n 



Sixteen pairs of mesodermic somites No. 5.">1 



Seventeen pairs of mesodermic somites No. r>(\S. 



Nineteen pairs of mesodermic somites No. .502 



Twenty-one pairs of mesodermic somites No. 55S 



The embryos were cut into transverse sections and reconstructed bv 

 the Born method at a maa-nification of two hundred diameters. Casts 



