70 EDWARD MCCRADY, JR. 



uninterrupted series at The Wistar Institute showing their 

 transition to the more familiar condition. 



In external views by transmitted light these thin closing 

 plates appear as very light areas as shown at the level D in 

 the totomount and section in figure 20. In this particular 

 series (17153) the second branchial pouches have not quite 

 met the ectoderm (fig. 20, E). This illustrates the cephalo- 

 caudal progression in the formation of the pouches, which is 

 shown in further detail in figure 31. More details of the 

 origin and development of these pouches will be discussed in 

 connection with the lung anlagen in stages 25, 26 and 27. 



The hypophyseal plate and the pharyngeal membrane. As 

 the subcephalic wrinkle, first observed in stage 23, pushes 

 farther beneath the medullary plate it makes two contacts 

 which are of permanent significance. The first is a contact 

 of ectoderm with ectoderm, the ectoderm of the wrinkle with 

 the infundibular depression in the medullary plate. The in- 

 fundibular depression is seen from a dorsal view in figure 20, 

 I, from a ventroposterior view in K, and from a ventroan- 

 terior view in L. In the last figure particularly, it is easy to 

 see how the ectoderm in turning back upon itself in the mid- 

 line will come to touch first the infundibular depression, and 

 second, the minute endodermal diverticulum which is the 

 first trace of the foregut. The plate which touches the in- 

 fundibular depression will later form Rathke's pouch and the 

 anterior lobe of the pituitary. That which touches the endo- 

 derm forms the pharyngeal membrane which separates the 

 stomodeum or buccal pouch from the pharynx until stage 29 

 when it becomes resorbed. 



Proamnion. The sixth point of secondary contact of ecto- 

 derm and endoderm mentioned above is the so-called pro- 

 amnion a crescentic region extending around the cranial part 

 of the embryo as far back as the vitelline plexus. It appears 

 in reflected light as a dark region (fig. 19, A) surrounding the 

 head. This structure, which is identical with that seen in an 

 8^-day, 3.4-mm. rabbit, or in a 4-somite, 23-hour chick, and 



