The origin and histogenesis of the thymus in Raja batis. 449 



placode and its rupture into two portions, instead of the original 

 "contact" there are two in connection with certain cranial ganglia, 

 one above the cleft and more or less on the level of the noto chord, 

 the other at the posterior margin of the cleft, and later more ven- 

 trally still. For a time from both ganglionic elements may be pro- 

 liferated. 



While, therefore, as Antipa showed, Froriep's ventral "contact" 

 with the thymus-placode is merely a topographical relationship during 

 a portion of the development, and never a union of the two, there 

 are two unions with the sensory epithelium. Further consequences 

 may be drawn. Frorief, probably rightly, identifies the ventral union 

 — and the real ventral union he failed to see — with that originally 

 described by him in mammalian embryos, and (p. 63) he states the 

 dorsal one to be absent in mammals. 



This absence in mammals is explained by the history of the dorsal 

 "contact" or placode in fishes. As is now well known, it furnishes a 

 portion of the lateral sense organs and one or more dorsal or supra- 

 branchial nerves. These sense organs and their nerves are not re- 

 presented in mammals (Ewart), and thus, apparently, not laid down in the 

 embryo. The ventral contact or placode must be present in mammals, 

 because of the persistence of that sensory portion of the postbranchial 

 nerve derived from it. Even in Torpedo or JBo/a, except in one arch, 

 the hyoid, this sensory part of the postbranchial nerve does not now 

 innervate sense organs. Perhaps originally it did, hence the mode of 

 development. In one arch it still does so, in the hyoid arch, and in 

 the skate there arises in this way the thickest of all its nerves, the 

 hyomandibular, which innervates what is really a ventral series of 

 sense organs, the hyoid group of ampullae^). 



From the existence of this double fusion with sensory placodes 

 and from that of a ventral series of sense organs on one arch a 

 further conclusion may be drawn. In a valuable piece of research 

 Miss R. Alcock (1898) has furnished details as to the arrangement and 

 innervation of the sense organs of the head of Ammocoetes. In this 

 she has shown, that here they are very simply arranged in a dorsal 

 and ventral series, innervated by dorsal (suprabranchial) and ventral 

 branches of the cranial nerves, VII, IX, X^— X*^. From the standpoint 

 of the comparative anatomist her conclusions have been criticised by 



1) For purposes of argument the mandibular offshoot of this may 

 be ignored. 



Zool. Jahrb. XVII. Abth. f. Morph. 29 



