666 FRANCIS M. BALDWIN 



Bdellostoma; some think it represents a rudimentary gill-pouch, 

 or that it is an ancestral gland of some sort; while others think 

 that it is formed merely by continued vegetative growth of the 

 branchial entoderm. 



In the elasmobranchs it develops behind the sixth pouch, and is 

 sometimes paired and sometimes single. In the amphibians it 

 usually develops behind the fifth pouch, and is usually paired 

 (anurans) or may be single (urodeles, Necturus excepted). In 

 Bombinator, according to Greil, '05, it is not developed on either 

 side. In the Lacertilia, it is sometimes single (van Bemmelen 

 '86), sometimes paired (Maurer, '99), while in the snakes, it may 

 be entirely absent (van Bemmelen) . The relative position of its 

 origin is a point upon which some differences are expressed ; some 

 authors interpreting it as developing from the pharynx behind 

 the fourth pouch, and others, behind the fifth. In the birds and 

 the mammals also, great diversity of opinions are expressed as to 

 its position, significance and fate, but since certain phases of this 

 have already been discussed (see pp. 657-661), a repetition here 

 is not necessary. 



Epithelial bodies and parathyreoids. In all the gnathostomes 

 thus far studied (elasmobranchs excepted, and if Mrs. Thomp- 

 son's '10, contention be correct, in these forms as well), certain 

 structures occur, which Maurer ('88) discovered in amphibians, 

 and to which he gave the name 'epithelial bodies,' the homologues 

 of which, according to certain authors, have since been discovered 

 in higher forms, but described under different names; parathy- 

 reoids (Sandstrom), glandules thyreoidea (Gley), et cetera. 

 These structures develop from the ventral region of the third and 

 fourth visceral pouches in the higher vertebrates, (reptiles, birds 

 and mammals) , and their homology to the epithelial bodies of the 

 amphibians become at once apparent. Although enbryologically 

 and anatomically quite distinct and different in every way from 

 the thyreoid, yet topographically, in the higher forms at least, the 

 parathyreoids become intimately associated with the former. 

 This fact is strongly emphasized by Mrs. Thompson ('10, p. 127), 

 who says, ' 'Thyreoids and parathyreoids are to be looked upon 

 as structures of somewhat different embryological origin, which 



