-26- 



as the only noticeable change from the preceding and much younger 

 stage, except increase in size, is the remarkable coiling of the 

 posterior end of the gland, caused, Dohrn says, as has been pre- 

 viously mentioned, by the longitudinal growth of the gland be- 

 ing more rapid than the space in which it is enclosed. The 

 larva from which this stage was taken had reached a length of 

 about 15 cm., and a circumference of about 2,b cm. in the region 

 of the thyroid. 



Fig, 6 j is a ventral view of the anterior end of a 

 larva of this stage, the ventral integiuaent (b.w.) of whiori has 

 been dissected along the mid-ventral line and dra\m to each 

 side, so as to expose the thyroid gland (thy.). The figure shows 

 the shape of the gland as seen from the ventral side, and also 

 its relative size and po8itir>n. The cartilaginous bars (c.b.) 

 of the branchial basket are also shovm, and it is seen that the 

 large mid-ventral bar divides, when it reaches the posterior end 

 of the gland, into two lateral bars, one of which lies close 

 to each side of the thyroid throughout nearly the entire length 

 of the gland. The longitudinal grooves (ventral and lateral in- 

 vaginations) are quite indistinct because of the mass of white, 

 fibrous-looking connective tissue which now surrounds the gland 

 on all sides. At this stage, as is seen in Pig. 6 j, the" gland 

 extends from the middle of the first to the middle of the fourth 

 gill pouch (g.p.) and is about 1/3 the diameter of the entire 

 pharynx. 



