625 



strong doubt on the interpretation of the ultimobranchial body as a 

 vestigial remnant of a once active gland, or epithelial organ of unv 

 character. It indicates rather that it is but the expression of a con- 

 tinued growth activity in the branchiogenic entoderm ; that it persists 

 for a time and in the growth transformations and shiftings of the 

 region becomes fused with the thyroid within which, in all prob- 

 ability and in accordance with all the evidence I believe, as Grosser 

 has stated, it speedily disappears through disorganization of its ele- 

 ments. That now and again all or portions of it may persist as epi- 

 thelial cords, tubules, or cysts, sometimes containing as inclusiuns 

 products of its protoplasmic activity as colloid, is 1 believe fully to 

 be expected, and is indicated by the evidence presented by Grosser. 

 Such occasional persistent epithelial structures derived from the caudal 

 pharyngeal complex need not necessarily possess a teological signifi- 

 cance ; that is, indicate a specific function, either past or present. 

 Even though it were found that from the complex colloid-filled 

 vesicles regularly were formed, there would be no clear indication 

 that the products of their metabolism necessarily produced in the 

 body the same effects as those of the (median) thyroid. 



Furthermore, it seems altogether likely that now and then all or 

 part of tHe epithelial remnant of the caudal pharyngeal complex 

 remaining after the differentiation and separation off of the para- 

 thyroid IV, may undergo thymic transformation, forming thus the 

 thymus bodies occasionally found without or enveloped within the 

 thyroid attached to or free from the parathyroid IV. I have earlier 

 in this paper stated my belief that there are no sufficient reasons for 

 deriving such a 'Thymus IV' exclusively or necessarily from the 

 ventral pocket of the fourth pouch. 



The morphological transformations which the caudal pharyngeal 

 complex nudergoes I interpret as but an expression of the growth and 

 growth shiftings of the regions, and as such may be illustrated by a 

 series of diagrams (figure 9) based on the embryos studied and 

 illustrating in outline the form relations from a stage preceeding the 

 appearance of the ultimobranchial body (3 mm.) until the parathyroid 

 IV is completely separated off (21 mm. +). 



No extensive bibliography is included or historical review pre- 

 sented as the classical work of Verdun '98 and the recent article by 

 Grosser '11 render such unnecessary in this brief note. 



As a summary it is only necessary to state again, more concisely, 



Anat. Anz. Bd. 47. Aufsätze. 40 



