232 Henry Fox 



form characteristic of the present stage, when the prjEcervical mass is 

 being finally constricted into two separate parts. 



As this is the latest stage in the series of pig embryos which I have 

 examined, I can state nothing as to the future history or fate of the 

 praecervical body in this animal. It .seems probable from its large size 

 that it would be present at birth. Kastschenko observed it in an 80 mm. 

 pig. Prenant claims that it is present at birth in the sheep. 



In cats of 23.1 mm. and 31 mm. I was imable to find any certain 

 traces of a fundus prscervicalis. This fact indicates that in this animal 

 the later behavior of the organ must be less complicated than in the pig. 

 It is probable that it rapidly disappears.^ 



In two late stages of the rabbit I could find only extremely uncertain 

 traces of the fundus. In a twenty days' foetus a slight process is present 

 at the dorsal apex of the left carotid gland. In a twenty-one days' 

 example this has apparently disappeared, but some distance above the 

 gland and entirely disconnected from it is a minute lymphoid body. 

 This may possibly represent the transformed process seen in the twenty 

 days' individual. However, this point cannot be settled until additional 

 material is examined. 



Review and Comparisons. 



The third pharyngeal pouch when typically developed closely resembles 

 the second. Like the latter, it has a prominent ventral diverticulum. 



The pouch becomes transformed into the thymus. The greater part 

 of the latter, i. e., its thoracic portion and cervical cord, are formed by 

 the downgrowth of the ventral diverticulum. 



The carotid gland is a derivative of the dorsal portion of the pouch. 

 It arises as a series of follicular outgrowths from the anterior wall of 

 the latter. 



The pouch does not separate entirely from the ectoderm, but remains 

 attached to that of the sinus prsecervicalis. Separation from the super- 

 ficial ectoderm takes place by the deepening and subsequent constriction 

 of the sinus, which accompanies the pouch in its passive withdrawal 

 from the surface of the body. 



The connection of the pouch with the pharynx is at first formed by 

 a wide opening. Later this is reduced to a solid cord, which subse- 

 quently constricts, thus leaving the pouch as an entirely independent 

 body, 



^Verdun asserts that it had entirely disappeared in an embryo of IG mm. 



