SCHULTE AND TILNEY, NEURAXI8 IN THE DOMESTIC CAT 345 



SO that they are slower in producing an effect upon the modeling of the 

 neuraxis. There is besides some evidence in the cat that somites are 

 added at the cephalic end of the series, which also would serve to explain 

 the retardation of the corresponding myelomeres. We are assuming that 

 the segmentation of the neuraxis into myelomeres is ontogenetically 

 secondary to the segmentation of the mesoderm, a view which receives 

 support from the fact that the number of myelomeres always lags behind 

 that of the somites, as well as by the fact that where the somites are 

 small and possibly retarded in appearance developmentally as in the 

 region just considered, there also the myelomeres are late in appearing. 



'it is possible, therefore, to recognize two principles of segmentation 

 in the deuterencephalon ; the first incident to the formation of the 

 cranial ganglia, the second associated with the segmentation of the 

 mesoderm, for it is to be noted that the appearance of the ganglionic 

 segments long antedates the branchiomeric segmentation. When, there- 

 fore, the series of myelomeres becomes continuous with the ganglionic 

 segments the result is not a meristic series of equivalents but comprises 

 structures diverse in their genesis and heterogeneous in their products. 

 In the stage of fourteen somites, the boundary between these two 

 series is gradually effaced and important changes supervene in the gang- 

 lionic segments. The first and second of these become subdivided ven- 

 trally. This is initiated in the first segment at the stage of fourteen 

 somites, in the second segment at nineteen somites. The third main- 

 tains itself as a small dilatation immediately in front of the first somite. 

 These changes coincide with the formation of the pontine angle, the 

 surface of which is marked by five elevations corresponding to the third 

 ganglionic segment and the subdivisions of the first and second. In 

 addition a small prominence, corresponding to the first myelomere, is 

 situated immediately caudal to the last of these elements and is also 

 recognizable at twenty-one somites (Plates XXXVII and XXXYIII). 

 If we now add to our reckoning the pre-ganglionic segment abutting 

 upon the posterior isthmian sulcus, a total of seven elevations is reached 

 for the hindbrain, a number within the limits of the count given by 

 students of the region in mammals, variation in which might well depend 

 upon the age of the embryo studied. 



It would seem, therefore, that these elevations correspond to the 

 neuromeres of authors. We have endeavored to show that they are 

 secondary and heterogeneous. 



