748 LIFE : OUTLINES OF GENERAL BIOLOGY 



ing as the larvae and the adults. For there is great plasticity in the 

 processes of cleavage, and the embryo adapts itself readily to the 

 tricks of the experimenter who isolates or disarranges the segmenta- 

 tion-cells. Thus, in illustration of plasticity, it may be noticed that 

 the third or equatorial cleavage, which divides four equal segmenta- 

 tion-cells into four smaller cells or micromeres and four larger cells 

 or macromeres, results in three difterent arrangements of the eight 

 cells — radial (as in some Echinoderms) , bilateral (as in Tunicates 

 and Cephalopods), and spiral (as in Polyclads and Annelids). These 

 three types are connected by every conceivable transition. 



After explaining that it is necessary to shake the eggs gently if 

 one would succeed in isolating blastomeres, and that there is a risk 

 of being led astray by broken fragments of blastomeres, Wilson 

 gives an account of the induced forms reared from isolated segmenta- 

 tion-cells. It will be convenient to speak of the first two blastomeres 

 as half -blastomeres, of the first four as quarter-blastomeres, and so on. 



An isolated half-blastomere undergoes cleavage identical with, or 

 approximating to, that of a normal embryo, forming a normal 

 blastula, gastrula, and fairly normal larva, each a half of the usual 

 size. 



Dislocation without separation of the two half -blastomeres results 

 in variously disposed double embryos like Siamese twins. 



An isolated quarter-blastomere often varies in its cleavage more 

 or less widely from that of an ovum. It may give rise to a blastula 

 and gastrula, differing from the normal only in size, but a segmental 

 embryo was rarely formed, and a larva with a notochord, neural 

 canal, and mesoblastic somites was only once obtained. 



A four-ceU stage, separated into two pairs of cells, may give rise 

 to two perfect half-sized dwarfs. Incomplete separation of the 

 quarter-blastomeres gives rise to double embryos, triple embryos 

 (one twice the size of each of the others), and rarely to quadruple 

 embryos. The double and triple forms may attain to the gastrula 

 stage; the quadruple forms did not rise beyond blastulse. In three 

 cases gastrulae of about one-eighth the normal size were formed 

 from shaking the four-celled stage, probably from a broken quarter- 

 blastomere. 



An isolated one-eighth-blastomere, which may be a macromere 

 or a micromere, segments in a manner approaching that of a complete 

 ovum, but never identical with it. In rare cases blastulse were formed, 

 living for two to three days; usually flat or curved ciliated plates, 

 like fragments of epiblast or hypoblast, were formed, which swam 

 about actively, and lived for twelve to eighteen hours. Dislocation 

 of the blastomeres at this stage gave rise to a great variety of forms : 

 the isolated blastomeres of the i6-cell stage gave rise to numerous 

 forms, including minute blastulse. Usually flattened plates or shape- 

 less masses resulted. 



