562 



is uninterrupted, and after the heart becomes enclosed, the outgrowth 

 extends to the anterior wall of the pericardium 



Fig. 6 shows a sagittal section through the thyroid outgrowth 

 iij an embryo 13 mm in length. At m the endoderm touches the 

 ectoderm where the mouth is soon to break through. The posterior 

 part of the oral fusion has now disappeared. From the floor of the 

 alimentary canal, the thyroid outgrowth extends backwards as a solid 

 bar of tissue to the anterior wall of the pericardium with which its 

 posterior cells come into intimate contact. 



pr th mh '2 m 



Fig. 6. Sagittal section through the thyroid outgrowth in au embryo 13 mm long. 

 alt alimentary canal. m mouth, mh Anlage of the mylohyoideus, pc pericardium. 

 tk, thyroid outgrowth. 2 — 2, anterior limit of fig. 7. 



Fig. 7 is from two sections in a series of horizontal sections 

 through au embryo 15 mm long. This figure extends as lar forwards 

 as the line 2 — 2 in fig. 6, and shows the relation of the thyroid 

 outgrowth to the branchial clefts. The section cuts the base of the 

 hyomandibular pockets, and one sees that the thyroid outgrowth still 

 extends backwards from the first, and not from the second branchial 

 pocket as stated by Maurer (loc. cit.) for T riton and Sire don. The 

 yolk granules have nearly disappeared from the mesoderm of the 

 anterior part of the head in the embryo from which the figure is 

 taken. The walls of the alimentary canal, however, are still filled 

 with yolk, and the cells composing the thyroid ontgrowth have hardly 

 fewer granules. 



The muscles sterno-hyoidei have developed on the outer wall of 

 the pericardium, and lateral to these is seen the anterior extremity 



