376 



"The place where the cephalic end of the notochord terminates 

 in this embryo is the region of Seessel's pocket, though a distinct 

 pocket is not observed in this preparation, and it is a question whether 

 this pocket exists in the human embryo. It has been shown that 

 the notochord ends in Seessel's pocket in chick, rabbit and sheep 

 embryos in early stages of development. The same seems therefore true 

 in the human embryo. The fact that in Embryo A of this series the area 

 of contact of notochord and pharyngeal epithelium, indicating the region 

 of development of the pharyngeal bursa, and the extreme anterior 

 point of contact in the region of Seessel's pocket may both be made 



Fis. 6. After Mead. 



out and separated by practically the whole length of the pharyngeal 

 vault would seem to me to suffice to controvert the contention of 

 Meyer that the pharyngeal bursa develops from Seessel's pocket." 



" In the mammals, the embryos of which are more generali}- acces- 

 sible for laboratory work, only the pig has a pharyngeal for bursa or its 

 homologue, and only in pig embryos does the head notochord reach the 

 retropharyngeal region and come in contact with the pharyngeal epi- 

 thelium." 



Now in my specimen, the head notochord does reach the retro- 

 pharyngeal region and come into contact there with the pharyngeal 

 epithelium. It does not come contact wath the pharynx wall at any 

 other point, but lies at slightly varying distances from it. In Professor 

 Huber's diagrams I cannot find Seessel's pocket and the posterior 



