Mosaic Development ui Ascidiaji Eggs. 221 



19. A possible explanation of the fact that all the fragments of 

 an immature egg may give rise to entire larvae, whereas the pro- 

 portion which gives rise to larvae steadily decreases after matura- 

 tion and fertilization, may be found in the fact that before matura- 

 tion the localization of ooplasmic substances is usually concentric, 

 after maturation and fertilization, polar-radial; while just before 

 or shortly after the first cleavage the localization may become 

 bilateral. Also the fact that isolated blastomeres may give rise 

 to w^hole embryos in some animals and to partial ones in others 

 may be due to the varying relations of cleavage planes to localiza- 

 tion planes. If at the close of the second cleavage the localization 

 is still radially symmetrical, each of the first four blastomeres 

 would probably be capable of giving rise to an entire larva; if the 

 first cleavage invariably lies in the plane of bilateral symmetry, 

 as in ascidians, each of the first two blastomeres might be capable 

 of giving rise to an entire larva (though my work shows that this 

 would not necessarily happen); if the cleavage planes do not 

 coincide with the planes of localization, as in moUusks and anne- 

 lids, isolated blastomeres would not give rise to entire larvae. 

 (See Wilson, '04s '04^) 



20. The development of ascidians is a mosaic work because 

 there are definitely localized organ-forming substances in the egg; 

 in fact the mosaic is one of organ-forming substances rather than 

 of cleavage cells. The study of ctenophores, nemertines, anne- 

 lids, mollusks, ascidians and amphibians (the frog) shows that the 

 same is probably true of all these forms and it suggests that the 

 mosaic principle may apply to all animals, (cf. Fischel, Wilson.) 



LITERATURE CITED. 



Barfurth, D., '93. — Halbbildung oder Ganzbildung von halber Grosse? Anat. 



Anz., viii. 

 Beneden, van et JULIN, '84. — La segmentation chez les ascidiens dans ses 



rapports avec I'organization de la larve. Arch, de Biol., v. 

 BovERi, Th., '01. — Ueber die Polaritat des Seeigeleies. Verb. Phys. Med. Ges., 



Wiirzburg, xxxiv. 

 '01.- — Die Polaritat von Ovocyte, Ei und Larve des Strongylocentrotus 



lividus. Zool. Jahrb., xiv. 

 Bracket, A., '04. — Recherches experimentales sur I'oeuf de Rana fusca. Arch. 



de Biol., xxi. 



