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



at last the lateral plates, the mesodermic visceral arch, into the inner 

 adductor and the outer constrictor muscles, with which the cartilaginous arch 

 is invested. 



The remainder of the mesectoderm constricted off from the rudiment of 

 the cartilaginous visceral arch is stretched at the same time to the outside of 

 the rudiment of arch so as to line the whole inner surface of the ectoderm 

 ventral to the chorda level, and assumes the characteristic feature of the 

 subcutaneous tissues which underlie the ectoderm, except the ventral part for 

 the hypoglossal muscles. This differentiation of the mesectoderm is very 

 obvious in larva? of the 13th to the 14th day. 



The mandibular arch, which we may now consider, is characterised 

 particularly by the enormous mesodermic arch which it contains and which 

 is folded upon itself, thrust inwards by the invaginating stomodasum, so that 

 four layers of the folded lateral plates are obvious on a cross-section through 

 this arch. While the ventral edge of the folded mesodermic arch, by which 

 the somatic layer passes over into the splanchnic plate, is separated by the 

 bottom of the stomodseum from its counterpart on the opposite side, the 

 dorsal edge, by which the two layers of the lateral plates also run into one 

 another, is divided only by the carotid artery from the chorda. The ventral 

 division of the mesectoderm, following this folding of the mesoderm, is 

 brought into the same topographical relations to the stomodaeum and to the 

 chorda. 



The dorsal edge of this division of the mesectoderm is brought into contact 

 with the lateral wall of the chorda, above the carotid artery, as is very clearly 

 seen on the 9th to 10th day. On the 13th day the cells composing this 

 edge are concentrated into a characteristic compact mass which, is soon 

 constricted off from the remainder of the layer. This compact cell mass 

 constitutes the earliest traces of what are known since Sewertzoff (97) as the 

 anterior parachordals. The rudiment of the anterior parachordal is on 

 cross-sections wedge-shaped and looks as if produced from the lateral wall of 

 the chorda. 



On the 14th day the rudiment of the anterior parachordal can be traced as 

 far as the branching of the facial artery from the carotid, which marks the 

 boundary between the first and second somites, and it ends backwards rather 

 suddenly in front of the roots of the vascular mandibular arches and of the 

 carotids. The parachordal rudiment is most prominent at a little distance 

 from the root of the vascular mandibular arch and grows gradually lower 

 toward the snout, while it is decreased suddenly in height backwards. 



The parachordal rudiment cannot, therefore, be distinguished genetically 

 from the rudiment of a cartilaginous visceral arch ; both the structures are, 



