26 Edmund B. Wilson. 



but never one that could be mistaken for a typical apical plate. 

 In others, however, the apical ectoderm does not differ from that 

 by which the whole pre-trochal region is surrounded. I feel 

 justified therefore in the statement that the lobeless larvae typically 

 fail to develop the apical organ at any period, individuals having 

 been reared up to the fourth day, when the metamorphosis of the 

 normal larvae was well advanced. {Cf. Figs. 31 and 34.) Dur- 

 ing the development, probably owing to the deficiency of material 

 present in the D-quadrant, the trochoblasts often become more or 

 less displaced towards the posterior pole, and in greater or less 

 degree lose their regular arrangement. In many specimens never- 

 theless the typical prototrochal belt of three rows of cilia is formed 

 (Fig. 34) , though even in these the rounded posterior region often 

 also bears patches of cilia. In others no definite belt can be made 

 out, and such individuals often give the appearance, when alive, 

 and even after being killed with formol, of being ciliated over the 

 whole posterior region. In preparations, however, the cilia of 

 such forms may almost always be seen to be arranged in patches, 

 leaving non-ciliated regions between them, which are doubtless 

 occupied by cells derived from the second and third quartets. 

 It is probable, therefore, that the appearance of uniform clllation 

 is misleading, and is caused by the confusion of separate tufts 

 lying at different levels. In cases where no displacement of the 

 trochoblasts occurs, the posterior region is covered by cells de- 

 rived from the second and third quartets. 



As the development proceeds there is no attempt to regenerate 

 the missing post-trochal region or apical organ, and the later 

 history of these larvae differs totally from that of the normal 

 ones. The pre-trochal region shows an increase, instead of a 

 decrease, in size, and is not withdrawn Into the interior, but gives 

 rise to a more or less irregular vesicular structure directed for- 

 wards as the embryo swims. Such larvae were reared until the 

 beginning of the fourth day (Fig. 34), after which they in- 

 variably became more and more irregular and finally disinte- 

 grated. At this period they present a most remarkable contrast 

 to the normal control larvae of the same age. There is still no 

 trace of a post-trochal region, no shell, no foot, and no apical 



