240 PROCEEDINGS OF THE ACADEMY OF [March, 



111 the figures (particularly figs. 17 and 18 [leftside]) one will 

 notice that the distal extremity is removed some distance from the 

 skin, but this appearance, I consider, is simply produced by the 

 obliquely ventral direction taken by the dorsal border, as has been 

 already described. 



This brings us to our third problem, i.e., the more pronounced 

 anterior extension of the fold. This, I consider, is correlated with 

 the growth anteriorly of the two hyoidean muscles. Naturally as 

 these extend forward they carry the fold with them. As a result 

 of this the posterior surface of the fold comes to face outward, and 

 the anterior inward. Hence in transverse sections structures 

 external to the fold are also morphologically posterior, whilst 

 those internal to it are morphologically anterior (compare trans- 

 verse with coronal sections of present stage). 



Stage IV. — Young tadpole. Opercular fold well developed, 

 ending freely posteriorly and with the ends of the external gills 

 protruding beyond its posterior margin. The various tissues for 

 the most part clearly differentiated. True cartilage developed in 

 the mandibular and hyoid arches (PI. VIII, figs. 15, 19; PI. IX, 

 figs. 23, 24). 



Beginning anteriorly the distal, blind extremity of the " diver- 

 ticulum " appears as a transversely extended cord of cells, some- 

 what expanded distally, lying in the loose mesenchyme some dis- 

 tance below the external epithelium (fig. 24, Tt/m.). This cord is 

 clearly distinguished from the surrounding fibrous tissue by its 

 greater density, which naturally causes it to stain more deeply, 

 and also by the presence within its substance of yolk spherules and 

 numerous pigment granules, similar to those found in the mucous 

 membrane of the pharynx. In the present stage the yolk spher- 

 ules, although still present, are much less numerous than in the 

 earlier stages and they soon disappear altogether, so that the dark 

 pigment becomes the distinguishing feature of the cord. The 

 lower proximal portion of the hyomandibular fold can be seen in 

 the figure as a shallow protrusion (JLjm.) from the ventre -lateral 

 angle of the pharynx (compare with figs. 10 and 11). In the 

 region immediately posterior the proximal portion is practically 

 blended with the wall of the pharynx (figs. 15 and 19). In fig. 

 19 (right side) it again becomes distinguishable and soon becomes 

 continuous with the prominent diverticulum JEii. (left side of fig. 19 ). 



