188 E. H. NORRIS 



I wish to thank Dr. Scammon for his valuable aid and criticism 

 and for the loan of material from his embryological collection. 



2. MATERIAL AND METHODS 



The present paper, based upon an extensive series of embryos 

 of Squalus acanthias, gives an account of the development of the 

 thyreoid from the time of its earliest appearance until the adult 

 structure is attained. 



The following table, in which the embryos are arranged in the 

 order of their lengths, shows the specimens used in this work. 

 A large part of the material is from the Harvard Embryological 

 Collection (H.E.C.), while some is from the Embryological Col- 

 lections of the Department of Anatomy of the University of 

 Minnesota (M.E.C.), the personal collection of Dr. R. E. Scam- 

 mon (S.C.), and the collection of the Department of Zoology of the 

 University of Kansas (K.U.E.C.). The material marked M.E.C., 

 as well as the older new-born specimens, were obtained by Dr. 

 Scammon from the Harpswell Biological Laboratory. My thanks 

 are due for the loan of material. An asterisk (*) following the 

 collection number signifies that the cervical region of the embryo 

 alone was sectioned. Otherwise the entire embryo was available 

 in serial sections. 



The ordinary reconstruction methods, both plastic (Born's 

 waxplate method) and graphic, were utilized in the present 

 study. In all cases where a determination of the follicular form 

 or structure was attempted, special precautions were observed 

 in making the reconstructions as accurate as possible. 



The drawings for reconstruction were made with the camera 

 lucida or with Edinger's projection apparatus on transparent 

 paper. After the drawings were completed, those of successive 

 sections were superimposed upon a tracing-table, and the cor- 

 responding structures in each section were then given a letter or 

 number. The drawings were controlled throughout by careful 

 microscopic observations, to determine the frequently complicated 

 relations of neighboring follicles. By this method it was possible 

 to determine with certainty the limits of any particular mass or 

 follicle. 



