Mervin T. Sudler 



41^ 



A- 



two lateral lobes are pyriform in shape and attached to the isthmus at 

 the smaller end. 



Mall "^ has noted that in the chick and dog the branchial arches 

 appear sooner on the left side than on the right and suggests that the 

 development of the heart and bending of the head to the left is the 

 cause of it. . Attention has been already called to a similar condition 

 in human embryos in this article, and it seems well to sum up the 

 differences which seem distinctive of the two sides at the different 

 stages. 



The statement of Sir Astley Cooper in regard to the thymus usually 

 being higher on the right side than on the left has been referred to 

 before. This same condition has shown 

 itself in these models. The thyroid, 

 however, is the most marked example. 

 In all of the models old enough to 

 show a division of the median rudimen,t 

 into the two lobes, the right one has 

 been the shorter and the left one the 

 longer, except in the embryo of the 7th 

 week where the two were about equal 

 in this respect. The lateral thyroid 

 rudiments are unlike and their differ- 

 ences are most striking in embryo 

 CLXXV where the right is still at- 

 tached to the pharynx and the left one 

 has lost its connection. The relative 

 positions and differences of the visceral 

 pouches of embryos XII and II have 

 already been explained. Taking these 

 differences together they show that in the human embryo the left side 

 of the pharynx develops slightly more rapidly than the right side in the 

 first few weeks of development. In the oldest embryo (embryo XXII of 

 the 7th week) these differences seem to have disappeared and the two 

 sides are sj-mmetrical or very nearly so. 



SUMMAKT, 



The shape of the human pharynx changes from a more or less rounded 

 cavity and one without many distinguishing characters, at the end of 



■Xl^-^.. 



Fig. 13. Ventral view of the dor- 

 sal part of the model of embryo 

 XXII. L., rudiment of larynx ; Oe., 

 oesophagus; Thym., thymus; Thyr., 

 thyroid; Tr., trachea. 



2^F. Mall: The branchial clefts of the dog with special reference to the orio-in of 

 the thymus arland. Also, Development of the Eustachian tube, middle ear, tym- 

 panic membrane and meatus in the chick. Studies from the Biolog-ical Laboratory 

 of the Johns Hopkins University, vol. 4, No.'-t, 18S8. 



