THE EMBRYOLOGY OF THE OPOSSUM 121 



not exist. The membrane referred to is composed of endo- 

 derm and somatic mesoderm, as I have already mentioned. It 

 is illustrated in the reconstruction in figure 39. Selenka 

 doubtless inferred the presence of the ectodermal sheets from 

 the composition of the edges of the amniopore in stage 28, but 

 he forgot momentarily that during stage 29 the mesoderm and 

 its coelom push around the cranial edge of the amniopore, 

 and separate the ectoderm at the edge of the head fold from 

 the endoderm. The nearly closed amniopore in my recon- 

 struction (fig. 39) shows the two layers of ectoderm which 

 he had in mind, but they are some distance caudal to the 

 lamella, and the latter is composed only of endoderm and 

 mesoderm. 



With this trifling exception Selenka 's account of all these 

 membranes is extraordinarily good, and it was partly with a 

 view to giving him credit for this achievement that I chose 

 one of his beautiful figures for a frontispiece. 



The pharyngeal bursa. The reconstruction of the pharynx 

 and f oregut in figure 39 J, shows a curiously gland-like ex- 

 tension of the extreme anterior tip. This structure is variable 

 in shape, number of branches, and extent of lumen. It occurs 

 only in stage 29, which represents roughly some 8 hours of 

 development, is always connected with the anterior end of the 

 notochord, and has no known function. Its principal interest 

 lies in the fact that it appears so suddenly, attains such con- 

 spicuous size, and disappears so quickly. 



Selenka described it as ' die Gaumendriise ' and then again 

 as 'die Gaumentasche.' He noticed its relation to the end of 

 the notochord, decided that it represents the anterior noto- 

 chordal canal, and that it is really not glandular in spite of 

 appearances. 



Before Selenka the following authors had described the 

 pharyngeal bursa in man: Mayer (1840), Lushka (1860), 

 Dursy (1869), Kolliker (1879), Froriep (1882). Of these, 

 only the last recognized the causative part played by the noto- 

 chord in this connection. Others had supposed the bursa to 

 represent either Seesel's pouch or Rathke's pouch. As 



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