444 JOHN BEARD, 



VON KuPFFER has happily termed the sensory placode, was by me named 

 the "branchial sense organ". Looking now at text-figure H (page 447), 

 I must still hold, that just as the thymus-placode is one component 

 of the complex termed a gill-arch and cleft, so the sensory placode 

 is another. And, exactly as the thymus- placode was perhaps originally 

 a structure, whose primary function was the production of leucocytes 

 for the protection of the two half-gills from bacteria etc., so the 

 sensory placode had originally some function of a sensory kind in 

 connection with the said gill-arch and gills. 



For a time after 1885 the system of the branchial sense organs 

 encountered little opposition. But in 1891 Froriep (1891, p. 60 — 65) 

 from investigations upon Torpedo-embryos came to different and 

 erroneous conclusions. His embryos, at least those treated of in the 

 paper, were mainly of late periods, and possibly they did not form a 

 continuous series. As a preliminary to the description of his own 

 observations, he writes : "Beard, welcher nach ihm [i. e. Van Wijhe] 

 die Selachier wieder untersuchte, hat die Kiemenspaltenorgane nicht 

 bestimmt von den Seitenorganen unterschieden, sondern beide zu- 

 sammengeworfen." 



He then proceeds to furnish proof of this by the account of his 

 finds in Torpedo, beginning with an embryo of — 12 mm ! In his text- 

 figure 5 it is shown for one of the branchial nerves, that it is con- 

 nected with epidermal thickenings at two points, one above the level of 

 the notochord and labelled by Froriep "Seitenorgan" (lateral sense 

 organ), the other immediately dorsad of the cleft and termed by him 

 "Kiemenspaltenorgan" (gill-cleft organ). 



In Froriep's sense the observation is incorrect, as Antipa after- 

 wards pointed out, but there is a fusion of nerve and epithelium in 

 this region, as Antipa also recognised. At certain periods the 

 branchial ganglion or nerve — speaking in a wide sense — is united 

 to the epiblast, i, e., to placodes within this, at two points. This 

 holds for certain of the arches at least, thus, in particular for the 

 hyoid and first branchial. In passing, it may be noted, and detailed 

 reference thereto reserved for a later page, that the same "Ver- 

 handlungen" contains observations of von Kupffer's of twofold con- 

 nections of cranial nerve or ganglion with the skin in Ammocoetes. 



Froriep's dorsal "contact" is in fact concerned in the formation of 

 lateral sense organs, as was recognised by him. The ventral one he 

 brings into connection with the thymus, and writes: "der Umstand, 

 dass bei Selàchierembryonen die Thymusbildung von dem Kiemen- 



