.IV 



COELENTEEATA 



97 



into a normal planula, and that in all cases one of the first four 

 blastomeres could do so. From such a blastomere in Clytia a hydroid 

 was reared, and in Ziriope a fully -developed medusa with four 

 tentacles was reared. 



Now Driesch and Morgan (1895) made a number of experiments 

 on the eggs of Beroe of which the most interesting are these. By 

 means of a fine scalpel they cut pieces from the unsegmented egg. Such 

 mutilated eggs mostly died, but about sixteen out of five hundred 

 survived, and eight of these developed into larvae with a diminished 

 number both of ribs and endodermal pouches. 



From this experiment Driesch and Morgan draw the conclusion 

 that the material required to 

 form definite regions of the 

 embryo is localized in definite 

 parts of the egg. This con- 

 clusion is amply confirmed by 

 the result attained from separa- 

 tion of the first blastomeres of 

 the egg by means of a scalpel. 

 Each of the first two blastomeres 

 produces an embryo with four 

 ribs. Each one of the first four 

 blastomeres forms a larva with 

 two ribs only; and further, when 

 one blastomere is separated 

 from the first four the remain- 

 ing three blastomeres form an 

 embryo with six ribs. 



Eischel (1897-1898) im- 

 proved on Driesch and Morgan's 

 methods : he separated the 

 blastomeres from one another 

 by subjecting the egg to pres- 

 sure, and by pinching the egg- 

 membrane with forceps. By this means he found he could make one 

 embryo produce three smaller embryos with a lessened number of 

 ribs, and he found that the united number of their ribs amounted 

 to the total of a normal embryo. By pressure he also separated 

 the smallest micromeres, i.e. those which give rise to the apical 

 plate, into two portions, and from these resulted an embryo with 

 two apical plates. When pressure was applied in later stages the 

 result was, not to produce several larvae with a lessened number of 

 ribs, but to break up the ribs already formed into several pieces. 

 In larvae with four ribs, produced from one of the first two blastomeres, 

 three endodermal pouches are formed, not two as one would expect 

 (Fig. 79). But Fischel points out that whereas the first two are 

 produced by the ingrowth of taeniolae in the tentacle plane, the 

 small third one owes its origin to the fact that the stomodaeum 



VOL. I H 



Fig. 79. — An emtryo of Beriie ovata with four 

 ribs and two endodermal pouches, and a 

 small extra third pouch ; obtained by isolat- 

 ing one of the first two blastomeres of Beriie 

 ovaia. (After Fischel. ) 



cTid, one of the two normal endodermic pouches ; 

 end% the small extra endodermic pouch ; r, the four 

 ribs. The other letters as before. 



