IV. i INITIAL STRUCTURE OF THE GERM 229 



LITERATURE 



H. E. CKAMPTON. Experimental studies on Gasteropod development, 

 Arch. Ent. Mech. iii, 1896. 



H. DRIESCH. Betrachtungen fiber die Organisation des Eies und ihre 

 Genese, Arch. Ent. Mech. iv, 1897. 



E. B. WILSON. Cleavage and mosaic work. Appendix to Crampton's 

 paper on llyanassa, Arch. Ent. Mech. iii, 1896. 



E. B. WILSON. Experimental studies on germinal localization. I. The 

 germ regions in the egg of Dentalium, Journ. Exp. Zool. i, 1904. 



E. B. WILSON. Experimental studies on germinal localization. 

 II. Experiments on the cleavage mosaic in Patella and Dentalium, 

 Journ. Exp. Zool. i, 1904. 



10. ASCIDIA. 



As has already been shown, there is in the Vertebrata and 

 Amphioxus no constant relation between the first furrow and 

 the sagittal plane of the embryo, although the symmetry of the 

 embryo does stand in a very definite relation to that of the un- 

 segmented ovum. Further, experiment has proved that the 

 isolated blastomeres, up to a ceitain point at any rate, are capable 

 of giving rise to normal total larvae. It is, perhaps, all the 

 more remarkable that in another group of the Chordata, the 

 Ascidians, the symmetry of the embryo is not only predetermined 

 in the symmetry of the egg but also by the planes of segmenta- 

 tion, and that the potentialities of isolated blastomeres remain 

 strictly what they would have been in the whole ovum. 



The earliest experiments are due to Chabry (Figs. 140, 141). 

 According to this author the first furrow in the egg of Ascidiella 

 aspersa is meridional and coincides with the sagittal plane ; the 

 second, likewise meridional, coincides with the transverse plane; 

 the third, equatorial, separates oral from aboral regions, and is 

 therefore horizontal. 



Isolated \ blastomeres or f right or left blastomeres give rise 

 to imperfect larvae with only one fixing papilla, only one atrial 

 invagination and one pigment-spot, the eye ; the last is absent 

 in the f left larva. % anterior larvae have one fixing papilla, one 

 sense- spot, a gut, a cerebral vesicle and a posterior notochord ; 



