DEVELOPMENT OF THE OALIFORNIAN HAG-FISH. 275 



measures 6 cm. in length, and will (probably) shortly hatch. 

 It exhibits occasional writhing, and its mode of growth sug- 

 gests that in tlie process of hatching the operculum will be 

 detached by movements of the tail. The colour at this stage 

 is purplish, save in the region of the yolk-sac. This, as before, 

 is bright yellow, closely traversed by vitelline blood-vessels. 

 The yolk-sac is now attached to the embryo only in the middle 

 portion of its trunk ; both the anterior and posterior trunk 

 regions seen in the figure are separate, therefore, from the yolk- 

 sac, although pressed tightly against it, and embedded in deep 

 grooves. The tail lies on its right side, and is growing over 

 the head toward the opercular end of the egg. The head, 

 lying somewhat on its left side, has now grown backward till 

 it has nearly reached the posterior pole of the egg. Struc- 

 turally this late embryo closely resembles the adult; cirri 

 are present around the mouth as in the adult condition, 

 although they cannot be seen in the figure, the mouth being 

 pressed far backward on the ventral side. The anus is indi- 

 cated at A, segmental mucous pits at m. Dermal-fin rays are 

 well developed. The gills, gs, are now the characteristic 

 pouches with infolded walls and with external tubular ducts. 

 Their final position will be seen to be far backward of the head. 

 It is evident, accordingly, when figs. 4, 5, 6, and 8 are con- 

 trasted, that the gills have not been growing forward at an 

 equal rate with the head region. They early arise near the 

 first slit even in front of the auditory sacs (fig. 4) ; they are 

 seen later (fig. 5) in the process of being drawn forward under 

 the head, but are now well behind the ear capsules ; still later 

 (fig. 6) they are seen in their normal position on the ventral 

 side of the throat, but further back behind the ear sacs, sepa- 

 rated from them by a distance of several somites; and finally 

 (fig. 8) they appear in their adult conditions, still further 

 caudad, now separated from the ear sacs by ten or more 

 somites. By the process of unequal growth, therefore, the 

 change in the position of the gills is explained.^ As at no 



1 As has also beeu shown to be the case in Amphioxus bj Lankester and 



Willey. (Editor.) 



