62 



THE EARLY DEVELOPMENT OF MAMMALS. 



bryo has a rhomboidal form which is complicated, however, by certain irregularities. 

 These are due to the formation of the gill-pouches, of which there are four distinct 



car. I. 



Hy. 



car.in 



pairs in mammals. Some authorities maintain that the ancestors of 

 had five pairs, and that the pair lost was situated between the present third and 

 fourth pairs (compare the remarks on "Zimmermann's arch," page 101). Each pouch 

 is a lateral pocket of the fore-gut, having a tapering form, the apex of which comes 

 into contact with the ectoderm. At the point of contact, entoderm and ectoderm 

 fuse to constitute a closing plate, similar to the oral and anal plates. In aquatic 

 vertebrates the closing plates are lost, and each gill-pouch becomes a true gill-cleft. 



The positions of the closing plates 

 soon after their formation are marked 

 by an external depression, the ecto- 

 dermal gill-pouch (Figs. 89 and 94). 

 The columns of tissue in front of the 

 first pouch, behind the last, and be- 

 tween the first and second, the second 

 and third, and the third and fourth 

 are known as the five branchial 

 arches. The first arch is called the 

 mandibular, the second the hyoid. 

 In each branchial arch an aortic arch 

 is developed (see page 99). In mam- 

 mals each pair of pouches has a 

 characteristic form in the embryo and 

 a characteristic differentiation. In the 

 12.0 mm. pig the first pouch (Fig. 28, 

 I), has a broad base, tapers toward 



apex rises 



the second 



IV. 



Bu. 



FIG. 28. PIG EMBRYO OF 1 2 MM. SERIES 518. OUTLINE the ectoderm, and its 

 OF THE PHARYNX AS SEEN FROM THE DORSAL SIDE. j , u j , . , 



FROMAWAXMODELBYA.R.KILGORE. tOWard the dorsal Slde 



I, II, in, iv, Gill-pouches. 2, 3, 4, 5, Aortic arches, pouch, II, occupies a more horizontal 



Ao, Aorta. Bu, Bursa pharyngis. car. in, internal plane and in form somewhat resembles 



arte 01 !"' ^J^f -^n" T ^^^ the first, with which it is partially 



artery. Hy, Hypophysis. Oe, (Esophagus. X 22 



diams. merged. The third pouch, III, is 



much smaller and is expanded at its 



end by a prolongation downward and inward; the prolongation has a somewhat 

 tubular form and extends far toward the aortic end of the heart; in the dorsal 

 view of the model it does not show. The fourth pouch, IV, is much smaller than 

 the others; it resembles the third pouch in having a ventral prolongation, but is 

 quite variable in form. 



The entodermal epithelium of the second to fourth pouches exhibits certain 

 specializations. One type is illustrated by the tonsil and thymus the epithelium 

 is thickened, assumes a reticular structure, and its meshes are invaded by leucocytes. 



