GIANT LARV.E FORMATION IN ARBACIA PUNCTULATA. 23 



In the preceding as well as in many other examples, the 

 grafted pair of eggs developed first into a double blastula and 

 later into a single gastrula. All subsequent development was 

 single. In all of these instances it was impossible to determine 

 the axis of the second member, for only one blastopore and one 

 gut was formed. It is possible that the eggs, in these instances, 

 were by chance grafted to each other in the same relative posi- 

 tions as the blastomeres of an egg, and that as a result of this 

 position the grafted pair developed into a single organism as 

 required by Boveri and de Hahn. That this possibility is very 

 remote will be shown in the following sections. Let me first 

 draw attention to such instances in which the axes of both 

 members are known. 



SINGLE LARV.E, AXES OF THE Two GRAFTED MEMBERS KNOWN. 



In this section the axis of each member was definitely estab- 

 lished by the definite formation of a gut in each member. 



Fig. 12 is a foreshortened view of a nearly equal pair of fused 

 gastrulse whose embryonic guts and therefore whose axes are 

 about 135 degrees apart. This double gastrula developed into a 

 "single" larva (Fig. 13) which grew into a larva decidedly 

 larger than the controls (Fig. 14). This larva contains certain 

 accessory parts which are not uncommon in fused larvae, at 

 least in certain stages of their development. There is, for ex- 

 ample, a small accessory oral rod at X, which structure is some- 

 times found in control larvae. There is also an accessory fold 

 of the gut, making four in place of three characteristic divisions 

 of the gut. This condition is very unusual if not entirely 

 absent in true single larvae. This example is but one of a series 

 which differ only in minor details and which show that "single" 

 giant larva may be formed even when the axes of the two grafted 

 members are not parallel and not symmetrical. How the two 

 guts are moulded into one, and how the relative position of the 

 axes is changed will be considered later. 



Whether the relative position of the axes is or is not perma- 

 nent, one would expect from Boveri's and de Hahn's hypothesis, 

 that two members, which were clearly not symmetrical, in 

 the specimen shown in Figs. 12 to 14, would not develop into a 

 single larva, which is contrary to our observations. 



