613 



larg;e and the sequence of stages so close that a fairly complete history 

 was thus obtained of the transformations in this region. The obser- 

 vations of Hammar '01 and Tandler '09 in particular leave little to 

 desire in a description of the earlier morphological transformations 

 that the fourth pouch and the related entodermal structures undergo. 

 These structures are so intimately associated in their origin and sub- 

 sequent fate that it is convenient for descriptive purposes to group 

 them together as constituting the "caudal pharyngeal complex", — a 

 term quite similar to those employed by Rabl '13 and by Grosser '11. 

 My study has therefore been mainly one of confirmation as covering 

 this period. In the matter or interpretation I believe that there is 

 ground for some divergence of opinion. This would not include, 

 however, any questioning of the essential "branchial" character of 

 the structure under consideration, as opposed to a ' postbranchial ' 

 value. Whether or not ' ultiraobranchial ' is as a name preferable to 

 'telobranchial' is a question that will be briefly referred to sub- 

 sequently. 



In addition to the two alternative interpretations above discussed, 

 there is the original view of Born and others that the structure in 

 question ("lateral thyroid") is a derivative of the fourth pouch, as 

 well as the ingenious interpretation of His, that the " lateral thyroids " 

 are added to the fourth pouches from the floor of the pharynx, coming 

 originally from the same anläge as the ' median ' thyroid. 



The development of the ultimobranchial body from the branchio- 

 gen ic portion of the pharyngeal epithelium is shown to be correct 

 from the relations in the younger embryos, and the conditions of 

 growth that lead to its appearance. In the youngest embryo examined, 

 whose length is estimated as nearly 3 mm. (2.2 mm. as sectioned) 

 and of whose pharynx and related ectoderm a model was made, three 

 pouches on either side reach the ectoderm with slight corresponding 

 ectodermic furrows. The second pouch on the left side is open. The 

 fourth pouch has not reached the ectoderm on either side. Behind 

 the third pouch, there is a cul-de-sac of relatively large size clearly 

 embodying potentially both the fourth pouch and the ultimobranchial 

 body. Upon the right side the third pouch shares with this a common 

 sinus whereby both open together into the medial pharyngeal chamber. 

 Upon the left side the third pouch possesses a more independent 

 opening. The conditions in this embryo are very like those in a 3 mm. 

 embryo described by Broman '96, the pharynx of which Hammar '01 



