So-called Mesectoderm in Petromyzon. 



471 



I believe, serially homologous with one another. The prominent point of the 

 parachordal rudiment develops into a transverse bar of cartilage, which is, I 

 assume, the rudimentary remnant of the equivalent of its corresponding 

 visceral cartilage bar. This bar is, in the mandibular arch, reduced to its last 

 remnant, because it has been shut off' from the respiratory mechanism. The 

 further fate of the cartilage bar interests us in developing into that important 

 element of the primordial skull, which is known since Parker (83) as the 

 palato-quadrate ; the redevelopment of this rudimentary remnant into so 

 conspicuous an element of the cranial skeleton is due to nothing but the law 

 of " Funktionswechsel " first enunciated by Dohrn. 



The remainder of the mesectoderm detached from the rudiment of the 

 anterior parachordal is, in the mandibular arch as elsewhere, converted into 

 the subcutaneous tissues, which develop in distinction to those in other arches 

 not only beneath the ectoderm, the skin of the cheek, but also beneath the 

 epidermis of the stornodseum, the cover of the mouth cavity. 



As Gaup (06) remarks, the single origin of the parachordal, which 

 Koltzoff (02) maintains and Schalk (13) confirms, is incorrect. On the 

 contrary, the posterior parachordal of Sewertzoff (97) is represented in 

 reality by the medial horizontal process of the cartilaginous hyoid arch itself, 

 and the transverse process, which, according to Sewertzoff, is very similar to 

 an ordinary visceral bar in the following arches, is nothing more than the 

 visceral bar itself in the hyoid arch. 



The anterior and posterior parachordals are separated for a long time by 

 interposition of the large auditory capsule. Both the rudiments grow 

 respectively backwards and forwards to meet and be fused together with 

 each other only in a larva about thirty days old, in which the auditory 

 capsule retreats and is detached from the chorda. But for a long time the 

 transition is obvious, because both the rudiments are decreased in thickness 

 towards their point of meeting. 



The trabecula is formed in front of the root of the facial artery and the 

 rudimentary vascular arch in the premandibular segment ; its centre lies 

 close to this vascular root. From this centre it grows anteriorly along the basal 

 wall of the brain and over the posterior cerebral artery, which is the anterior 

 prolongation of the carotid artery. The more it is prolonged, the more it 

 diverges hand in hand with the artery from the median line, so that it lies 

 opposite the optic cup rather on the lateral wall of the brain and embraces, 

 with its counterpart on the opposite side from right and left, its infundibulum, 

 and is finally lost on the lateral wall of the latter. 



The mesectoderm cells giving rise to the centre of the trabecula are 

 doubtless those derived from the first somite, and are marked off from those 



