Mosaic Development in Ascidian Eggs. 193 



but they all illustrate the principle that a blastomere never gives 

 rise to any other structures than those which it would produce in a 

 normal embryo. 



7. Anterior and Posterior Half Gastrulce (Figs. JJ-Sz). 



In a recent publication Driesch ('03) has maintained that an 

 alteration in the capacity for regulation occurs in the ascidian 

 development between the early and the late gastrula stages. 

 When the open cup-shaped gastrulae of Phallusia were cut in two 

 transversely into anterior and posterior halves, each of these 

 halves developed into "einer vollstandigen kleinen Appendi- 

 cularie, welcher Organe niederer Bedeutung (Otolith, x'\ugenfleck) 

 eventuell fehlten." However, w^hen the elongated gastrulae w^ere 

 cut in two transversely a head developed from one piece and a tail 

 from the other, "so deutlich und sharf begrenzt und ausgebildet, 

 als babe man eine fertige Appendicularie scharf durchschnitten." 



Considering the results which I have obtained on the develop- 

 ment of the two anterior or two posterior cells of the 4-cell stage of 

 Cynthia the conclusions of Driesch seemed most remarkable and 

 I therefore undertook to repeat his experiments upon Cynthia. 

 Gastrula of the stage shown in Fig. 8 were cut in two with a sharp 

 knife made from a needle, under a Zeiss binocular dissecting 

 microscope. With the power used the individual cells of the 

 yellow crescent could be plainly seen and it was always easy to 

 determine the exact boundary between the anterior and posterior 

 halves. In every instance the section was made as close as possi- 

 ble to this boundary (second' cleavage plane) and so as to leave all 

 of the yellow cells in one of the pieces. Owing to the presence of 

 the chorion the experiment was not an easy one to perform, since 

 the chorion w^ould frequently slip under the knife, or the egg move 

 within the chorion. Nevertheless in one day I succeeded" in 

 cutting in two about thirty of these early gastrulae; ten of these 

 lived for twenty hours or longer after the operation, the others 

 were too badly crushed to survive. Four of these which survived 

 the operation are shown in Figs. 77-82, the drawings having been 

 made from nineteen to twenty hours after the operation. Every 

 one of these ten surviving embryos was a partial one and, although 

 I was unable to determine their structure with the same amount of 

 detail as m the case of stained and mounted preparations, it was 



