ORGAN-FORMING SUBSTANCES IN EGGS OF ASCIDIANS. 2O/ 



are not directly visible. Recent experimental work on some of 

 these forms confirms and extends these conclusions and proves 

 that even in the egg before cleavage begins different substances 

 may be present which are destined in the course of development 

 to enter into specific parts of the embryo. 



The most notable differentiations of the ooplasm which have 

 been observed hitherto are found in Myzostoina (Driesch, 1896; 

 Wheeler, 1897; Carazzi, 1904), in Stroirgyloccntrotus (Boveri, 

 1901), in Unio and Chcetopterus (Lillie, 1901, 1902), in Dcntalhnn 

 and Patella (Wilson, 1904) and in the gasteropods Crcpidnla, 

 P/iysa, Planorbis and Liinn&a (Conklin, 1902, 1903). In none of 

 these cases, however, are the differentiations and localizations of 

 the ooplasm so remarkable as are those which occur in the ascidian 

 egg. Here the different substances of the egg are strikingly dis- 

 similar ; they are localized in their definitive positions at a remark- 

 ably early period, and they may be traced with ease and certainty 

 through the maturation and fertilization, the cleavage, the gastru- 

 lation and the later stages until they give rise to specific organs 

 or parts of the larva. 



MATERIAL AND METHODS. 



I have studied the early differentiations of the egg in three spe- 

 cies of simple ascidians, viz., Cynthia (Sty da) partita Stimpson, 

 Ciona intcstinalis (L.) Flemming, Molgula manhattensis Verrill. 

 The differentiations and localizations are essentially the same in 

 all of these species, but as the different kinds of ooplasm are 

 more brilliantly colored in Cynthia than in either of the other 

 genera named, I shall devote particular attention to this form. 



In an extensive publication on the organization and cell-line- 

 age of the ascidian egg (Conklin, 1905), I have figured and 

 described the remarkable localization of germinal materials in the 

 egg of Cynthia. The differentiations of the egg substance are 

 here so great and their localization so precise that the figures and 

 descriptions of these might well seem to be exaggerated. I there- 

 fore welcome the opportunity of publishing a series of photomi- 

 crographs of these eggs, an opportunity which I owe to the skill 

 and courtesy of Misses Foot and Strobell. Their method of 

 photomicrography, which they have fully described in previous 



