STRUCTURE OF THE PHARYNGEAL BARS OF AMPHIOXUS. 17 



boundary of a primary slit ; the ccelom is here in the form of a 

 canal giving off shoots to each bar, and fro them appearance 

 presented in such a preparation, it is not an inconsistent inter- 

 pretation to regard the co2lom as sharing in the downgrowth 

 of its ventral wall. The appearance presented by the rod in 

 the hinder gill-slits is thus : M. 



D. Certain Abnormal Bars. 



In one series of sections the pharynx presented a few bars 

 containing sometimes three and sometimes two rods. In all 

 cases these abnormal bars are " primary " bars, i. e. the rods 

 contained within them are cleft rods, similar to those found in 

 normal primary bars; and on each side of such abnormal bars 

 there lies a normal tongue bar, separated by a pharyngeal slit 

 from the abnormal one. On Plate 7, fig. 27, I represent 

 one such bar cut at about the middle of its length, containing 

 three rods, which are in all respects similar to one another. 

 The bar itself is otherwise normal, and presents the usual 

 characters of the epithelium in its various parts. Naturally 

 the bar is much wider than an ordinary one, especially at its 

 atrial end. The septal membrane (s.m.) is very short, but the 

 two usual blood-vessels (vix.v. and som.v.) are present. There is 

 not a vessel at the apex of each rod, as one might expect ; the 

 coelom is very extensive, and dips in between the rods as the 

 nuclei show. The usual subepidermic blood-vessel (skel. v.) is 

 also present. But this bar, if traced upwards towards the 

 epibranchial groove and downwards towards the endostyle, 

 presents variations of this condition in different regions. 



At its origin dorsally the three rods are fused together, but 

 this occurs for only a very short space, and we soon tind three 

 rods. Towards its lower end two of these (Nos. 1 and 3) are 

 fused, so that the bar has but two rods ; and still further a 

 junction between these two is effected (fig. 28), and there 

 appear to be but one rod, rather larger than usual, but evidently 

 consisting of two united rods. Soon, however, these separate 

 again, and I take this union as representing a synapticulum. 



But now the relative position of the two rods changes ; 



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