36 Edmund B. Wilson. 



tion between apical organ and post-trochal region is direct or in- 

 direct — i. e,, whether the development of the one depends on that 

 of the other, or whether the development of the two is only con- 

 nected through their common relation to the polar lobe. Further 

 experiments conclusively show that the latter is the case; for in 

 several ways larvae may be produced that possess the apical organ 

 but lack the post-trochal region. My first experiment to test 

 this consisted in the isolation, separately, of the four micromeres 

 of the first quartet (la, ib, ic, id), which may easily be effected 

 by means of Herbst's calcium-free sea-water. The result of this 

 experiment, several times repeated, is that while all four of these 

 micromeres may develop into actively swimming ectoblastic em- 

 bryos, the one derived from the D quadrant ( id) , and this alone, 

 develops an apical organ (Figs. 49, 50) . All of these four small 

 embryos are of approximately the same size, ovoidal or some- 

 what pear-shaped in form, with a group of active trochoblasts at 

 the larger (posterior) end. The anterior region is covered with 

 fine cilia (as in the AB ^-larya or the A, B or C ^4 -larva) ; 

 but only the id larva bears in addition the characteristic apical 

 tuft, which is nearly or quite as large as in a whole embryo, and 

 is borne upon the usual ectoblastic thickening or apical plate. 

 None of these larvae gastrulate or develop a post-trochal region; 

 from which it follows that after the completion of the third cleav- 

 age not only is the development of the apical organ independent 

 of that of the post-trcchal region, but at this time the posterior 

 micromere of the first quartet, id, is already definitely specified for 

 the formation of that organ, independently of its relation to the 

 remainder of the embryo. The result of isolating the cells of the 

 4-cell stage is entirely in harmony with this, as already mentioned. 

 The A, B or C 34 develops into a closed pyriform larva swimming 

 normally with the smaller and turned forwards, but entirely 

 devoid of apical organ or post-trochal region (Fig. 48). The 

 D ^, on the other hand, though often distorted, shows typically 

 the apical organ, and an exaggerated and usually irregular post- 

 trochal region. (Fig. 47.) This result is in striking contrast 

 to the fact, mentioned above, that in Patella, each of the quadrants, 

 whether of the 4-cell stage or of the first quartet, may develop an 



