124 PROCEEDINGS OF THE AMERICAN ACADEMY 



dorsal wall of the cleft, just as in many young stages the mandibular 

 arch forms the sloping anterior wall of the cleft. 



After bisecting an embryo 8 mm. long, and laying open the pharynx, 

 the cavity of the first cleft may be viewed from the inside. It will be 

 found to be a deep blind pocket extending dorsally and outwards, the 

 opening to wliich is long and narrow, although somewhat wider near 

 the centre. The inner openings of the other clefts also are still very 

 conspicuous. 



In an embryo 11 mm. in length the opening of the first cleft has be- 

 come more diamond-shaped, and is sharply defined by a ridge which 

 forms its dorsal and posterior margins. The remaining clefts, having 

 become entirely closed, are now only recognizable as shallow furrows. 



When the embryo has grown to 16 mm. in length these furrows 

 have become almost obliterated. The ridge which forms the posterior 

 and dorsal boundary of the first cleft is more prominent. By carefully 

 reflecting it one may observe a bridge of tissue, the primitive tympanic 

 membrane, through which a certain amount of light penetrates. A 

 hair inserted from the outside through the middle of the floor of the 

 external portion of the cleft will pass through this membrane at a point 

 near its centre. From this it is evident that the external depression 

 must lie directly opposite the internal part of the cleft, and that there- 

 fore the former is not a secondary depression at one side (dorsal) of 

 the external part of the cleft, as claimed by Urbautscliitsch. 



An embryo 18 mm. long shows that the ridge still exists, and that it 

 serves to define the pharyngeal opening of the first cleft; there remains, 

 however, no trace of the other clefts. 



In order to study the shape of the internal part of the first cleft, as 

 well as the changes which it undergoes and its relation to the external 

 portion of the cleft, it is necessary to employ a series of frontal and 

 cross sections through tliis region at various stages of development. 

 For this purpose embryos were killed in picro-sulphuric acid, and hard- 

 ened in alcohol. They were stained in toto in Grenadier's alcohol- 

 borax carmine. Several other methods of hardening and staining 

 which I employed gave much less satisfactory results. 



Sagittal sections through the head of an embryo G mm. long show 

 that the walls which bound the first cleft in front and behind have 

 already come in contact along a line running nearly parallel with the 

 surface of the neck, but much nearer the superficial than the deep, or 



