296 SCOTT. ' [Vol. I. 



ob aber die klcine Grube am Boden des Vestibulum dem Re- 

 cessus sacculi und der sackformige Anhang der Schnecke gleich- 

 zustellen sind, will ich meines Theiles unentschieden lassen. 

 Ohne verbindende Glieder zwischen dem Gehororgan 

 der Petromyzonten und dem des iibrigen Fische scheint mir 

 eine Entscheidung dieser Frage sowohl wie im Ganzeii die von 

 einer sicheren morphologischen Deutung des Gehororganes der 

 Petromyzonten nicht moglich zu sein." 



Unfortunately the development of this organ throws but little 

 light upon these obscure homologies. The ear is the first of 

 the sense-organs to be formed in the embryo ; on the fourteenth 

 day after impregnation (Fig. 28, PL X) the epiblastic cells 

 lying near the anterior end of the hind-brain increase greatly 

 in height, but remain single layered, and are at the same time 

 depressed so as to form a shallow pit. The pit rapidly deepens 

 and is cut off from the skin, forming a large oval vesicle with 

 comparatively thin walls, and very long recessus labyrintJii (Fig. 

 29, PI. X; Fig. 40, PI. XI). At this stage the auditory vesicles 

 are so large as to form conspicuous prominences on the sides 

 of the head. Shipley has shown that about the twenty-second 

 day " certain patches of the epithelium become higher than the 

 others, and the cells develop each a very large cilium which pro- 

 jects into the cavity and bears a knob at its free end." These 

 cilia are not shown in Calberla's preparations (or, at all events, I 

 did not see them), but are perfectly clear in the beautiful sec- 

 tions which Mr. Shipley has so kindly sent me. In horizontal 

 sections these ciliated patches are seen to be confined to the 

 anterior and postero-internal portions of- the vesicle, and 

 apparently represent the rudiments of the cristcB acusticce. 



At this stage the vesicle is ellipsoid in shape, the fore and aft 

 axis being somewhat the longer, though but slightly so. This 

 difference becomes more and more marked with advancing de- 

 velopment, and in larvae 20 mm. long and upwards the trans- 

 verse axis is much the shorter; the vertical axis also increases 

 in length. When muscles make their appearance between the 

 ear-capsule and the skin, the prominences of the head disap- 

 pear, and the auditory vesicles are removed from the surface. 

 In larvJE of 8 mm. (Fig. 30, PI. X) there appear two processes 

 from the outer and inner walls of the vesicle which grow toward 

 each other. The inner process is situated higher up and just 



