1901.] NATURAL SCIENCES OF nilLADELPIIIA. 225 



I. Observations and Results. 



I now turn to the description of the development of the tjm- 

 pano-Eustachian passage in the common toad. In this undertak- 

 ing I shall first treat in detail the condition and relations of the 

 structures under consideration in the different stages, beginning with 

 the earliest, and then at the end of the paper summarize the chief 

 features of this development. 



Stage I (PI. VI, fig. 1). — I begin at a stage when the hitherto 

 almost spherical embryo has elongated and when the tail has grown 

 out as a short stump. No external gills are as yet apparent. The 

 head has become diffei'entiated from the boJy proper and the 

 region immediately posterior to it is marked by two or three slight 

 dorsi-ventral grooves, indicating the position of the future 

 branchial-clefts. 



PI. VI, fig. 1 is a coronal section of the anterior portion of an 

 embryo of this stage. The section is slightly oblique, the right side 

 being cut at a higher plane than the left. In this figure one will no- 

 tice that the anterior extremity of the pharynx is still separated from 

 the exterior, the conjoined eudoderm and ectoderm forming at 

 this point a solid partition of cells — the stomatodeal plate (si.). 

 From this region posteriorly the cavity of the pharynx gradually 

 widens out until it forms a spacious chamber, the sides of which 

 are marked by four dorso-ventral grooves, marking the inner 

 openings of the visceral-clefts. Just back of the fourth visceral- 

 cleft the cavity narrows very suddenly to form the lumen of the 

 oesophagus. 



As shown by the figure, there are only four visceral- clefts 

 {Hym., 2-4 v./.) marked out at the present stage. With the 

 exception of the fourth, each of the clefts extends outward as a 

 solid, double-layered plate of eudoderm, continuous at its inner 

 end with the epithelial lining of the pharynx and externally in 

 contact with the deeper layer of the ectoderm. Only the medial 

 portion of each cleft shows a lumen. The fourth visceral-cleft 

 resembles the others, except that it does not as yet quite reach the 

 epiblast. 



® In the drawing the distal extremities of the clefts are shown separated 

 by a narrow, clear area from this layer, but this coudiiion, I think, uiust 

 have been produced by shrinkage, a supposition which receives support froni 

 the rough and irregular character of the distal edge. 



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