614 



described, modelled and figured (liis figures 3, 4). The conditions in 

 the 3 mm. embryo also recall those described by Bremer in a 4 mm. 

 embryo (Harvard collection, No. 714) in which the third and fourth 

 pouches, — the latter of which at this stage has reached the ectoderm, — 

 open into a common pharyngeal recess. The description also suggests 

 that the caudoventral diverticulum (from the fourth pouch) which is 

 to become the ultimobranchial body is just making its appearance. 



In the embryo (4.9 mm.) described by Ingalls '07, it would 

 appear from the figure (No. 7) that the ultimobranchial body exists 

 as a marked diverticulum of the fourth pouch. The letter X by which 

 it is indicated is explained as the " free end of the fourth pouch ". 

 In the text the author states his inability to decide whether or not 

 this represents a "postbranchial body or the so-called lateral thyroid 

 anläge" (i. e., ultimobranchial body). 



In the third embryo of the series examined, 5 mm. in length, 

 the ultimobranchial body is well developed. Four pouches reach the 

 ectoderm. Upon one side (R) behind the contact of the fourth pouch, 

 the entoderm possesses a slight contact with the ectoderm, the con- 

 dition thus closely paralleling that of the 5 mm. embryo described 

 by Hammar '04. The mesoderm (mesodermic arch ?) between this 

 rudimentary fifth pouch, — if it may be so interpreted, — and the fourth 

 pouch is very slight and contains no demonstrable blood vessel. 

 Upon the left side, behind the fourth pouch the entoderm possesses 

 a small projection toward the ectoderm which is however very slight. 

 The caudal pharyngeal complex which exists as a broad diverticulum 

 of the pharynx possesses three portions, — an extension directed toward 

 the thyroid, or as probably more correctly expressed, toward the 

 bifurcation of the truncus aorticus, cephalo-ventrally and medially ; 

 the portion adjoining the ectoderm, and a marked caudo-lateral 

 extension ; the first usually designated as the ventral pocket of the 

 fourth pouch, or as Thymus IV, the last being the so-called ultimo- 

 branchial body. These two structures are closely on a line centering 

 at or near the thyroid. The relations are very similar to those in 

 embryo Ha 5 modelled by Tandler. The ultimobranchial body thus 

 does not share the general concentric aiTangement about the thyroid 

 and truncus aorticus shown by the branchial pouches I— IV, but 

 on the contrary the outpocketing has quite the reverse direction. 

 Nor is there any approach to the ectoderm, so that the ultimo- 

 branchial body as such cannot be considered a fifth pouch, nor 



