No. 2.] LINE SYSTEM OF BATRACHUS TAU. 239 



These thickenings of the ectoderm are described by H. V. 

 Wilson (14) as "sensory tracts," and he maintains that in the 

 bass, unlike what has been observed in selachians, in Amia 

 and in the trout, the lateral line originates in the form of 

 "sensory sacs," which later on become flattened out into the 

 " patches " described by other authors. There is no dissent 

 from the view that the auditory region is the place where the 

 lateral line system originates, but the occurrence of these 

 "sensory sacs" appears to be peculiar to the bass. Wilson 

 states that the ear, branchial organ, and the first of a series of 

 organs extending on to the body, are derived from this " common 

 sensory furrow." In Batrachus there is no definite furrow 

 present, and the "branchial sense organ" described as "func- 

 tional in the later stages of embryonic and during larval life" 

 is certainly not " histologically differentiated " as in the bass. 

 There is no sign of an organ composed of "sense cells with 

 short stiff hairs," as described by Wilson in an embryo of 

 fifty-nine hours. 



Fig. 17 shows the growing end of this line as seen in a 

 preparation of the skin of an embryo 5 mm. in length. After 

 fixing in picro-sulphuric acid and slight maceration in water, 

 if the skin is removed, stained in alum cochineal, and mounted 

 in glycerine, the proliferating cells in the lower layer of the 

 ectoderm may be clearly seen. A horizontal section of this is 

 shown in Fig. 18, at the time when the growing point has 

 reached only a short distance behind the pectoral fin. A 

 more highly magnified view of a portion of the same is seen in 

 Fig. 19. A comparison of the lateral line of Batrachus at 

 this stage with the same structure in an Acanthias embryo is 

 of interest (Fig. 14). In Acanthias the lateral line is quite 

 conspicuous. In an embryo of 22 mm. (Cut 3) it is easily seen 

 with a hand lens, as a prominent, somewhat flattened ridge, 

 extending backward above the branchial region and along the 

 sides of the body. There is a curious fold of the epidermis, 

 the so-called "pocket," which covers the growing end of the 

 line. PI. XIX, Fig. 14, shows a horizontal section of an embryo 

 17 mm. in length, from which it is evident that the "pocket" 

 consists of a reduplicature of the skin accompanying the 

 enlarged growing point. 



