226 H. D. Senior. 



than that used in the preceding figure). The parts shown^^ are 

 (normally) in immediate contact with the yolk, and represent the 

 region extending from the tip of the notochord back nearly to the 

 anterior limit of the first body somite. Formation of the ventral wall 

 of the pharynx, by the folding ventralwards of the (originally) lateral 

 region of the giit-entoderm on either side, is complete in the anterior 

 region and is rapidly extending backwards. The lateral plates are 

 following the medial margins of the ventral pharyngeal wall in their 

 progress toward the mid-line. 



The outline of the entodermal pharynx presents, on each side, 

 three prominences which require some explanation. Of these the 

 posterior is the hyoid ectodermal pouch which has now reached, and 

 blended with, the (very shallow) ectodermal pouch; this, for reasons 

 stated below/^ will be referred to as the hyo-branchial anlage. The 

 middle prominence is the mandibular entodermal (solid) pouch; 

 this blends with the ectoderm later but (as is well known) gives rise 

 to no cleft ; it undergoes disintegration soon after the perforation of 



'•■'Explauation of Figures 4, 5, G, and 7. 



Red, lateral plates, where these are tovered veiitrally by endocardium their 

 outline is indicated by a red line. 



Blue, endocardium. 



Continuous black liiir indicates the position of the outline of the entodermal 

 pharynx and adjacent part of the head-fold (see text) ; interruptions in this 

 line indicate blending of pharyngeal entoderm, or of head-fold, with the basal 

 layer of the surface ectoderm. 



Broken hJacJc line, medial margin of (closing) ventral wall of pharynx. 



>^tii)i)le, ventral wall of the entodermal pharynx and adjacent head-fold 

 ectoderm. 



Plain ahite, dorsal wall of (incomiilcte) pharynx. 



Arrows on each side indicate the longitudinal limits of the "descent area" 

 of portion moyenne. 



"Although the (solid) hyoid pouch is alone present at this stage the branchial 

 entodermal pouches (also solid) are about to be laid on. very rapidly, from 

 before backward. The hyoid and branchial entodermal pouches all reach 

 a common ectodermal anlage and are separated from one another by an ex- 

 tremely delicate partition of mesoderm so that, at any given time, a very care- 

 ful examination is necessar.v to determine the exact number of pouches actu- 

 ally present; in order to avoid a repeated analysis of the condition, which 

 is unnecessary for this investigation, the entire series of compactly grouped 

 pouches has been looked upon as a single structure. 



