DEVELOPMENT OF PETROMYZON ELUVIATILIS. 349 



canal remains in contact with the epiblast, and here the 

 gill opening subsequently appears about the twenty-second 

 day. 



Huxley was the first to point out that the embryo 

 Lamprey possesses eight gill-slits, and his account has been 

 confirmed by Scott and Dohrn, who, however, point out 

 that the first slit remains closed, and does not open to the ex- 

 terior, as Huxley described. Dohrn has further shown that the 

 first or rudimentary gill-slit becomes converted in the ciliated 

 groove encircling the mouth, which was first described by 

 Anton Schneider in Ammocoetes. 



Fig. 27 represents a longitudinal horizontal section of the 

 head of a twenty-one days' old embryo. The eight primitive 

 gill-slits are here shown lined by columnar epithelium, which 

 in the posterior seven is most flattened at those points where 

 the opening will subsequently appear. The corresponding area 

 in the first cleft, however, will be seen to be lined with very 

 high columnar cells. These cells afterwards acquire cilia and 

 come to lie in a deep groove. 



The branchial vessels have only appeared in the first gill- 

 bars, but the cells which will be converted into the cartilagi- 

 nous gill arches have already become distinct {br. b.). About 

 the twenty-second day a process begins to grow backward from 

 the middle of each gill-bar into the gill-slit behind. This re- 

 duces the slit to a <i-shaped opening. After the opening to 

 the exterior has been established the gill-bars overlap each 

 other, the passage from the cavity of the mouth to the exterior 

 being directed outwards and backwards. Each gill-bar 

 acquires a few gill filaments, into which the blood courses. 

 The whole is covered by a layer of thick columnar epithelium 

 continuous with that lining the rest of the mouth, except cer- 

 tain small areas, mostly at the end of the short filaments, 

 where the epithelium has become suddenly thin, thus putting 

 the blood into closer communication with the surrounding 

 water. 



The columnar glandular-looking cells which line so much of 

 the cavity of the mouth contain a number of very fine gran- 



