LATEE LARVAL DEVELOPMENT OF AMPHIOXUS. 207 



situation on the left side, has suffered retardation of develop- 

 ment. When these retarded slits do appear, they are called, 

 arbitrarily, the secondary slits. 



The secondary slits form to the number usually of eight 

 (sometimes seven and sometimes nine), above and alternating 

 with the primary slits on the right side, appearing long before 

 the completion of the atrium in front. The first secondary 

 slit always lies between the second and third primary slits, and 

 usually arises somewhat later than the succeeding six secon- 

 dary slits, which develop simultaneously. It will be shown 

 later that the simultaneous appearance of the secondary slits is 

 only due to a masking of the metameric mode of formation. 



As the secondary slits grow in size the primary slits gradu- 

 ally pass round to the left side, and meanwhile certain of them 

 commence to atrophy. It is especially noteworthy that after 

 Stage I the first primary slit becomes progressively smaller and 

 smaller, until by the end of Stage V it has disappeared alto- 

 gether. The view will be supported below that the closing of 

 the first primary slit has a profounder significance than the 

 closing of the hinder slits, or at any rate a different meaning. 



By the closing of some of the primary slits their number is 

 reduced to the same figure as that of the secondary slits, 

 whether it be seven, eight, or nine. The primary slits which 

 atrophy are, therefore, the following : 



a. When 7 primary slits persist — Nos. 1 and 9, 10, 11, 12, 13, 14 close. 

 i3. „ 8 „ „ „ 1 „ 10, 11, 12, 13, 14 „ 



y. „ 9 „ „ „ 1 „ 11,12,13,14 „ 



It is during the passage of the primary slits from the right 

 to the left side that the completion of the atrium is effected. 



The long axis of the secondary slits is at first parallel to the 

 long axis of the body, while at the same time that of the 

 primary slits is at right angles to the long axis of the body. 

 The axes are equalised as soon as the two series of slits are 

 properly adjusted on their respective sides, and then the slits 

 on each side commence to elongate in a direction at right 

 angles to the long axis of the body. 



VOL. XXXII, PAKT II. — NEW SER. O 



