6i2 PATTERNS AND PROBLEMS OF DEVELOPMENT 



terest. First, the polar (longitudinal) axis of the larva has been repeatedly 

 described and figured as transverse to the chief axis of the parent suctorial 

 body; second, separation and differentiation of the larval from the parent 

 body progresses transversely to the larval longitudinal axis. Two stages 

 of this type of larval development are shown in Figure igs, A and B. In 

 A the larval axis is almost parallel to the parental axis, and in B it is not 

 transverse. According to Collin, this position is merely an anomaly, but 

 it suggests the possibility that the larval axis may be primarily parallel 

 to, and determined by, the parental axis and that the transverse position, 

 even though it may be assumed or approached very early in development, 

 is actually secondary. If this is true, the larval pole bearing the disk of 

 attachment develops toward the distal pole of the parent; and if a gra- 

 dient is present in the parent body, the distal pole probably represents 

 its high end. As already noted, the larval pole of attachment is in advance 

 in locomotion, that is, it is physiologically apical. In Figure 193, C, of 

 another genus, the position of the fully developed larva with the pole of 

 attachment more or less proximally directed with respect to the parent 

 has doubtless resulted from locomotion after separation. The axis of the 

 second developing larva is oblique, not transverse to the parental axis; 

 and the figure suggests that a third larva may be developing below and to 

 the right of the second, with its axis still more nearly in line with the 

 parental axis. The progress of development transversely in the larva is in- 

 dicated in Figure ig^,A,B, and C; but in A and B and in the second larva 

 of C the physiologically apical pole of attachment appears to be slightly 

 in advance of the opposite pole as regards separation from the parent 

 body, that is, there is some evidence of an apicobasal differential. Collin 

 gives figures of other species showing the larval axis parallel or almost 

 parallel to the parental axis (Fig. 193, D, E).^ However, even if the larval 

 polar axis is primarily coincident in direction with, and a part of, the 

 parental axis, the question of the factors determining the change to trans- 

 verse position in certain species and the plane in which it occurs remains. 

 If it is primarily transverse to the polar axis, as it appears to be from many 

 descriptions and figures of certain species, the manner of its establish- 

 ment and the progress of larval development from one side of the body 

 to the other present puzzling problems. 



Larvae of some Suctoria develop from external buds. The larva of 



5 See also Collin, Figs. Ill/, LXXXIII, LXXXIV, XCI a, XCV e. Still other figures sug- 

 gest that the polar axis of the larval type of Fig. 192, D, may also be primarily parallel to the 

 parental a.\is. 



