750 PROFESSOR W. C. M'INTOSH AND MR E. E. PRINCE ON 



maxillae and maxillae develop in advanced embryos as paired translucent rods (PL XL 

 fig. 20), which gracefully curve, like bars of chitin, below the eyes forward to the 

 ethmoidal region, and form the sole lateral supports of the oral roof (PL X. fig. 1 ; 

 PL XIII. fig. 7). They are essentially superficial, and lie in a thin stratum of membrane 

 which stains deeply, called by Pouchet " tissue generateur," and occupying the situation 

 of Parker's " subocular bands," though he regards them as the rudiments of the pterygo- 

 palatine arch (No. 117, p. 113). The homology of these dermal maxillary rods, with the 

 labial cartilages of more primitive forms, as suggested by Dr Gunther (No. 61, p. 90), 

 is of much interest. A pair of curved bars, probably palatine elements, are also developed 

 in the roof of the oral chamber at a late embryonic stage. They are irregular in thick- 

 ness, slightly curved, and attenuated at the extremities (PL XL fig. 18). 



When first distinguishable, the pharyngeal bars consist simply of solid mesoblastic 

 thickenings passing along the lateral and ventral walls of the mouth, and more or less 

 oblique in direction ; moreover, in cross-section, these thickenings are found to be paired, 

 and united in the middle line, forming a roof over the pericardial chamber (PL XL 

 figs. 1-3, 6-8). The cells assume a columnar arrangement, and constitute laminae, 

 which appear as parallel superposed strata, when the bar is cut longitudinally (PL XL 

 fig. 9), but in a cross-section of a bar these strata are observed to be somewhat concentric 

 and laminated (PL XL figs. 6-8). Each rudiment of a branchial arch (fg, PL XL 

 figs. 6-8), when fairly defined, consists of a cylindrical mass of cells, concentrically 

 arranged round the central point of the bar, and limited above by the epithelial hypoblast 

 of the pharynx, and below by the pericardial hypoblast. They increase in length, and 

 change from the transverse to the antero-posterior oblique position (PL VIII. fig. 9), the 

 inner extremity of each pair of arches apparently shifting forward, so that they point 

 anteriorly (PL X. figs. 2, 3, 5); while their upper and posterior parts, which extend 

 up the lateral walls of the pharynx, have moved very slightly from their primary position. 

 Neither the mandibular nor the hyoidean arches are so markedly transverse in situation as 

 the branchial bars proper, and they alter very little in position as development proceeds. 

 In the gurnard, three days old, at the anterior end of the hyoid arches, i.e., where the 

 copula is formed, a large boss occurs, formed chiefly by a free development of the lining 

 membrane of the oral floor. This membranous expansion (really a lingual rudiment) pro- 

 jects as a large irregular elevation on the floor of the mouth, and is lifted up by the 

 erratic movements of the hyoid arch, as though the operation of deglutition were being 

 performed (PL XIV. fig. 2). Gradually the arches lose their dense indifferent appear- 

 ance, and become converted into cartilage, the small primary cells being broken down, so 

 that each bar consists of larger flattened elements placed transversely, and giving the 

 arches a transversely striated appearance (PL IX. fig. 5). The flattened cells become 

 hyaline, and each arch shows a single column of hyaline discs contained in a thin peri- 

 chondrial membrane. The first two arches wholly assume this character, and are seen to 

 be composed of these discs or chondroplasts placed one above the other along the 

 whole length of each bar (see the figure just referred to) ; but in the four arches which 



