GENERAL ACCOUNT OF DEVELOPMENT. 47 



eight hours after the eggs were laid, the little embryos work 

 their way out of the egg-membranes and swim freely. Their 

 condition at hatching is shown in Figs. 25 and 26, p. 59. 



After hatching, the embryos continue to develop rapidly, 

 and in about thirty-six hours from the time of spawning they 

 reach the stage shown in Fig. 34, p. 74. The mouth is not 

 formed until the end of this period, and development up to this 

 stage is apparently effected at the expense of the small amount 

 of food-yolk contained in the egg. 



After the formation of the mouth, the embryo continues its 

 pelagic life, but from this time develops slowly, increasing in 

 length, and gradually acquiring the shape and characters of the 

 adult. During this period the anterior part of the body presents 

 an extraordinary asymmetry, by which the mode of formation of 

 the g(ll-clefbs, which appear in order from before backwards, is 

 profoundly modified. The mouth is a large oval opening (Fig. 36) 

 placed, not on the ventral surface, but on the left side of the 

 pharynx. The gill-slits of the two sides appear, not simulta- 

 neously, but successively ; those of the left side, which may be 

 termed primary slits, being formed before those of the right side, 

 or secondary slits. The primary slits, of which there are as a 

 rule fourteen, are not at first on the left side, but in the mid- 

 ventral wall of the pharynx, and shift upwards so as to actually 

 lie for a time on the right side of the pharynx. The secondary 

 slits, usually eight in number, appear along the right side of the 

 pharynx dorsal to the primary slits (Fig. 38) ; while between 

 the two series of gill- slits, primary and secondary, the endostyle 

 is formed at the anterior end of the pharynx. 



During the later stages of pelagic life, the total duration of 

 which is about three months, this curious asymmetry is 

 gradually rectified. The mouth assumes its median position, 

 the primary gill-slits shift across the median line, and take up 

 their permanent position on the left side of the pharynx ; the 

 endostyle shifts from the right side to the mid-ventral wall ; and, 

 by disappearance of some of the primary gill-slits, the number 

 of primary and of secondary gill-slits becomes equalised, eight 

 being present on each side of the pharynx. 



At the close of the pelagic period, which may be called the 

 critical stage, the young Amphioxus, now about 3*5 mm. in length, 

 adopts the mode of life of the adult, burrowing in the sand, and 



