472 Mr. S. Hatta. Mesodermic Origin and the Fate of the 



of the second somite by the facial artery, which appears much earlier than 

 the mesectoderm. But the forward growth of the anterior parachordal seems 

 to be carried on largely at the cost of the dorsal division of the mesectoderm 

 from the second somite, viz., the cells which lie close to the chorda, and 

 to be continued uninterruptedly to the formation of the trabecula in front ; 

 so that the trabecula is practically not formed separately from the anterior 

 parachordal, as was believed by previous observers, but as the prolongation 

 of the latter. The trabecula has, however, a special centre for itself, marked 

 by a slight thickening in the rudiment. 



It is, however, still an open question, whether the trabecula is to be put in 

 the series of the visceral arches or not. But it is obvious that the rudiment 

 of the trabecula is genetically identical with the latter, because it is formed 

 of material identical with that giving rise to the visceral arches and because 

 it comes forth in a metamere identical with that which has a cartilaginous 

 visceral arcli for itself. 



To avoid misconception, a few words may be said about the relation of the 

 mesectoderm to the metamery of the body. The epithelial bands into which 

 the ventral division of the mesectoderm is divided are in numerical as well 

 as topographical correspondence with the mesodermic visceral arches, that is 

 to say, branchiomeric. The cartilaginous visceral arches are derived from 

 these branchiomeric bands, also branchiomeric in arrangement. 



The mesodermic visceral arches arise from two unsegmented continuous 

 layers, the lateral plates, which are divided by nothing but evagination of the 

 visceral pouches, which is independent of the process dividing the mesoderm 

 into the somites. This metameric repetition in the entodermal pouches has, 

 therefore, no direct relation to the mesodermic somites at all, which represent 

 the primary metamery of the body. In spite of its being the product of the 

 segmented mesoderm, the mesectoderm is also an unsegmented continuous 

 layer, until it is divided by the visceral invagination into branchiomeric 

 bands. The branchiomery in this part of the meso- and mesectoderm, which 

 is brought about in a passive way, is, therefore, not of the same value as the 

 body-metamery. 



The branchiomery is furthermore in segmental accordance with the ventral 

 series of the ectodermic placodes for the ganglia* But this series of ganglia 

 is never in direct segmental connection with the mesodermic somites. 



Still another organic system, which is in segmental accordance with the 



* This series of ganglia consists of the facial and glossopharyngeal ganglia and of the 

 series of the epibranchial ganglia. The placode for the lens may be put in this series, for 

 it can by no means be distinguished from the ganglionic placodes, so far as its origin and 

 mode of development are concerned. 



