IV. i INITIAL STRUCTURE OF THE GERM 195 



merely forked. The two guts may be similarly oriented or 

 turned in opposite directions, and it is interesting- to observe 

 that Driesch, who believes the first furrow to coincide with the 

 plane separating the two guts, accounts for the latter case by 

 supposing that one blastomere had been rotated upon the other^ 

 so that their vegetative ends the locus of the gut-forming 

 substance faced in opposite directions. 



Neither the ectoderm of the early gastrula alone nor the 

 endoderm is capable of giving rise to a larva, though the former 

 can develope a stomodaeum, a statement confirmed by Morgan. 



The vegetative half of a gastrula of Spliaerecldmis will give 

 rise to a normal small Pluteus whether the ectoderm only, or 

 together with it the tip of the archenteron, has been removed 

 (Fig. 104) ; the animal half will not. When the spicules, one or 



d . 



FIG. 104. The potentialities of the cells of embryonic organs ; a, b. 

 The vegetative and animal portions of a gastrula of Sphaerechimw 

 gmnularis, cut equatorially in two ; c. Pluteus reared from a fragment 

 of a gastrula ; d. Normal tripartite pluteus gut. (After Driesch, 1896.) 



both, are removed the skeleton is one-sided and the Pluteus con- 

 sequently one-armed. 



In Astenas the archenteron can form a new terminal vesicle 

 and mesenchyme cells afresh when these have been removed 

 (Fig. 105); but should the coelom sacs be cut off at a later stage, 

 the (secondary) archeuteron can form no more, though it divides 

 into the usual three portions. 



Parallel to the behaviour of isolated blastomeres is the segmenta- 

 tion and development of egg-fragments, obtained by shaking, 

 cutting, or (Loeb) dilute sea-water. The fragments may be taken 

 from already fertilized eggs, or from unfertilized : in the latter 

 case each, whether nucleate or enucleate, must be subsequently 

 fecundated. 



In segmentation there are a great many differences, which 

 appear to depend on the nature of the fragment, that is to say, 



o 2 



