100 PROCEEDINGS OF THE ACADEMY OF [Feb., 



gland lias appareutly reached its greatest complexit)', 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\aously men- 

 tioned, by the longitudinal growth of the gland being 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 cir- 

 cumference of about 2.5 cm. in the region of the thyroid. 



Fig. (y is a ventral view of the anterior end of a larva of this 

 stage, the ventral integument ( b. w. ) of which has been dissected 

 along the mid-ventral line and drawn 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 position. 

 The cartilaginous bai's (c. 6. ) of the branchial basket are also shown, 

 and it is seen that the large mid-ventral bar divides when it reaches 

 ttie 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 

 invaginations) 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 fig. 6/, the gland extends 

 from the middle of the first to the middle of the fourth gill -pouch 

 (g.p.), and is about one-third the diameter of the entire pharynx. 

 In a ventral view, the coil of the posterior end of the gland does 

 not of course show, but at each end is seen a median groove 

 which shows, even through the fibrous envelope ; these grooves are 

 caused by the separation of the two parts of the gland, at each 

 extremity, whereas throughout most of their length the halves are 

 in close contact with each other. The numerous transverse folds in 

 the seven pairs of gill-pouches are seen in this figure, and also the 

 fibrous character of the anterior wall of the pharynx. The actual 

 oral opening is not shown. 



Fig. GA; is a diagrammatic side view of the gland, to show the 

 points through which the sections were cut, and also to show longi- 

 tudinal space relations, such as size and position of duct, extent of 

 coil, etc. Being carefully reconstructed and drawn by scale, it 

 shows these relations with more or less accuracy. As is seen from 

 this diagram, the duct lies a little nearer the posterior than the 

 anterior end of the gland, though if the coiled posterior end of 



