A HUMAN EMBRYO OF TWENTY-FOUR PAIRS OF SOMITES. 



137 



short of reaching the ectoderm. A fourth pouch is seen extending from the 

 ventro-lateral surface of the pharynx. It arises to a certain extent in common 

 with the third, and shares with it the third ventral pharvngeal groove. It is 

 slightly pointed and is directed outward, backward, and downward. 



The remaining portion of the foregut, that is, that part between the fourth 

 pharyngeal pouch and the yolk-stalk, is shown in plate 3, figures 3 and 4. It 

 presents, above, a definite swelling which is apparent when seen in either front 

 or side view. Looked at from in front, the swelling appears double, a median 

 longitudinal groove separating two rather elongated protuberances. A cross- 

 section of this region is shown in text-figure 2. A short distance below this swelling 

 there is another which, when viewed from the side, is seen to be rather pointed. 

 It is just dorsal to the sinus venosus (text-fig. 3). Still farther caudally is seen 

 the hepatic diverticulum (text-fig. 5). 



ch.d. 



a.seg.d 



ao.d.s. 



gas.r 



a.seg.l. 



v.vitd. 



wild 



s.v. 

 Fie. 4. Portion of section 92. XlOfi diameters 



v.vits. 



cap.pl. 



Fir,, o. Portion of section 112. X H'i(i diameters. 



PULMONARY DIVERTICULUM. 



The significance of the two upper swellings I am unable to determine defi- 

 nitely from this specimen alone. Thompson 45 describes a somewhat similar condi- 

 tion in the Robert Meyer embryo of 23 segments. The upper swelling he inter- 

 preted as a bilateral pair of lung-buds; the lower as the beginning of the stomach. 

 Grosser 15 , however, from a study of this region of the same embryo, concluded that 

 Thompson's bifid swelling represents probably a fifth pair of branchial pouches, 

 and that the lower swelling is the lung-bud. 



Regarding Thompson's description he states: 



"Thompson (1907) asserts that in this embryo the lungs have a paired origin, but 

 he does not figure it, and this statement has been transferred to the Normentafel. But 

 he is clearly in error as to the place where the lungs develop, as shown by his own descrip- 



