FORMATION OF THE MOUTH. H 



Fig. 27) is soon modified, and in the next stages of development (PI. 

 II. Figs. 2, 4), the digestive cavity (d) no longer runs in the centre of 

 the larva, but is bent slightly to one side. If we examine one of the 

 embryos about forty hours old (PI. II. Figs. 5, G), we find that great 

 changes have taken place in the thickness of its walls. The outer wall 

 has everywhere become much thinner, except near the opening thus 

 far called mouth, where the decrease is not so marked. The walls of 

 the digestive cavity, which were of an equal thickness for the whole 

 length, have become exceedingly attenuated at the bottom of the sac, 

 and have dilated to a considerable extent, forming a sort of reservoir 

 with very thin walls at the extremity of the pouch (PL II. Figs. 4, 6, d, 

 magnified and isolated. Fig. 1, d). These changes in the thickness of 

 the walls, and in the form of the internal cavity, are also accompanied 

 by corresponding changes of form in the embryo as a Avhole. The ex- 

 tremity opposite the so-called mouth has increased in bulk, and greatly 

 exceeds in size the perforated extremity (PI. II. Figs. 4, 6) of the body. 



When seen in profile (PI. II. Figs. 2, 4, 5), still greater changes are 

 visible j there is a decided difference between the two sides of the em- 

 bryo, forming what is to become above and below ; calling that part 

 below, where the mouth is situated in the adult larvte, and which is car- 

 ried downward in its natural attitude while moving. The dorsal portion 

 of the larva projects beyond the so-called mouth, so that the perforated 

 extremity has become bevelled ; the narrowing of the central portion 

 of the larva has increased, and the digestive cavity which, in younger 

 embryos, occupies the centre of the cylinder (PI. I. Figs. 27, 28), is bent 

 towards the lower side (PI. II. Figs. 2, 4, 5, d). The outer wall has be- 

 come thickened at a point opposite the bent extremity of the digestive 

 cavity, and the thickening of the wall, together with the bending of the 

 digestive cavity, goes on till the closed end touches the lower side at m. 



The changes which have taken place during the time elapsed since the 

 twentieth hour have been very gradual. The embryo now enters into a 

 state where the changes are exceedingly rapid and important; so much 

 so that at the end of the third day the embryo has, in a rudimentary 

 state, all the parts characteristic of older, fully developed larvse. 



At the end of the second day the reservoir at the extremity of the 

 digestive cavity has changed its outline from a circular to a lobed one 

 (PL II. Fig. 8, o) ; the lobes widen towards the sides, almost forming 



3 



