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EMBRYOLOGY OF THE CHICK 



THE VISCERAL CLEFTS AND VISCERAL ARCHES 



Different parts of the embryo grow at different rates of speed, and 

 while the heart was formed directly under the anterior end of the diges- 

 tive tract on the second day, on the third day the heart has shifted its 

 position so far posteriorly that there is a distinct space between it and 

 the head proper. This space we may call the neck or pharynx. 



It is in this region that the mesoderm has not divided into the two 

 layers the somatic and splanchnic. We therefore still have a sort of 

 sheet consisting of the three primitive layers of ectoderm, mesoderm, 

 and entoderm, extending outward from the embryo. 



The entodermal lining of this neck region becomes pushed out into 

 four narrow pockets (Fig. 295, A), called the visceral or gill pouches, 

 during the latter part of the second or the early part of the third day. 

 These meet with ectodermal depressions formed as furrows which grow 

 inward to meet the gill pouches. The thin wall between the outpushings 

 and the ectodermal inpushings breaks through in the lower forms, such 

 as in fish and amphibia, and there remains open throughout life, but in 

 the chick the opening is seen in the first three pairs during the first three 

 or four days. It remains open for about two days. These openings, or 

 places where openings usually occur, are known variously as visceral 

 clefts, gill clefts, or branchial clefts. 



As the neck is considerably curved, these clefts do not lie parallel 

 to each other, but converge toward the ventral part of the neck. The 

 fourth cleft never opens in the chick. 



Numbering and naming of these clefts begins with the most anterior 

 and continues caudad. 



A 



Fig. 295. 



A, Horizontal, and B longitudinal section through the head region of Ammo- 

 coetes (larval stage of lamprey.) ao.b., anterior aortic arch; ao.d., dorsal aorta; 

 oo.v., ventral aorta ; di, invagination which separates the anlage of the thyroid 

 gland from the digestive tract; m, anlage of mouth; thyr, thyroid anlage; 1, 

 ciliated gill region which probably becomes the spiracle ; 2-8, gill pouches. 

 (A, after Vialleton ; B, after Dohrn.) 



