GIANT LARV/E FORMATION IN ARBACIA PUNCTULATA. 25 



(Fig. 1 8). During the next two days the non-developing member 

 continually decreased in size, and finally disappeared altogether; 

 its mate progressively differentiated and grew into the "single" 

 larva (Fig. 19) which is larger than control larvae. This example 

 is typical of a fairly large group in which one member is slowly 

 but completely absorbed and in some instances disintegrated. 



A stage between that shown in Figs. 17 and 18 is represented 

 in the next example. This grafted pair consists of a blastula 

 and a somewhat larger gastrula. A free-hand sketch is shown in 

 Fig. 20. During the next twenty-four hours the blastula dimin- 

 ished in size and gradually disappeared, while the gastrula de- 

 veloped into a normal young and single larva (Fig. 21). During 

 the next five days this larva enlarged considerably, accessory 

 skeletal bars and accessory gut appeared, as seen in foreshortened 

 view in Fig. 22. During the succeeding four days besides the 

 expected reduction in size of the larva, a number of accessory 

 skeletal bars also disappeared from both anal and arm rods, 

 making the larva resemble even more closely single control larvae. 



In the next example a blastula is grafted to a gastrula (Fig. 

 23). The blastula gradually diminished in size and disappeared 

 without further differentiation, while the gastrula grew and 

 differentiated into a "single" larva. This larva has a single gut 

 bent a little to one side, with a single skeleton plus an irregular 

 accessory rod connected to the basal and aboral rods of the 

 skeleton (Fig. 24). During the next four days, while the larva 

 continued to grow, two accessory parts of the gut appeared 

 making a five part digestive tract in place of a tri-partite gut 

 as in normal animals. The hind and mid guts are in duplicate. 

 The accessory bar is also enlarged and is much fenestrated. In 

 this example as well as in the preceding, one of the grafted 

 members clearly disappeared BUT DURING AND AFTER THE DIS- 

 APPEARANCE OF THIS MEMBER ACCESSORY PARTS APPEARED IN 

 THE SKELETON AND IN THE GUT OF THE DOMINANT MEMBER. 



Such accessory parts appeared in all or nearly all cases after 



one member disappeared, and was frequently associated with a 



general enlargement of the body. Accessory parts are not rare 



among controls, but they do not occur as frequently nor are they 



of the character, nor are there so many accessory parts in a single 



