230 



INTERNAL FACTORS 



IV. i 



in | posterior larvae the chorda and brain are absent, though the 

 three germ-layers are all present. 



In the segmentation of the isolated blastomeres the direction 

 of division is altered, in that a furrow (the fourth) which in the 

 whole egg is meridional and at an angle of 45 to the first two 

 becomes parallel to the plane of separation, and so resembles the 

 third furrow of the whole egg. 



A right or left \ blastomere or f , anterior or posterior, right 



FIG. 140. A. Egg of Ascidiella aspersa in the two-cell stage. One 

 ^ blastomere has been killed (G). B. The survivor (D) divides to form, in C, 

 a small normal gastrula. (After Chabry, from Korschelt and Heider. 



At. 



FIG. 141. Larva of Ascidiella aspersa produced from f blastomeres. 

 F, sucker ; At, atrium ; No, notochord ; En, endoderm. (After Chabry, 

 from Korschelt and Heider.) 



or left, blastomeres consist after this division of two tiers of four 

 cells each ; the cleavage of these cells resembles, therefore, that of 

 a whole egg. 



It should be noticed that the blastomeres cannot be actually 

 separated ; it is only possible to kill one, by a needle, and note 

 the development of the survivor. 



The more recent researches of Conklin on Cynllna have con- 

 firmed and extended Chabry's results. 



The immature egg of Cynthia (Fig. 142) comprises a central 

 grey yolk surrounded by a peripheral layer of yellow pigmented 



