178 Edwin G. Conklin. 



nor muscle cells. Unfortunately this particular embryo was not 

 followed through the various stages of development until it gave 

 rise to a larva and none of the older stages which I have studied 

 have shown precisely this type of injury, i. e., the destruction of 

 the yellow crescent without injury to the ectoderm cells of the 

 posterior half. ^ 



In many other cases which I have seen all of the posterior half 

 of the egg was injured in the 4-cell stage. I have followed the 

 development of the surviving anterior halves of such eggs as late as 

 the stage of the metamorphosis of the normal larvae; the develop- 

 ment of such blastomeres is always partial. Figs. 49, 50 and 51 

 represent three views of one and the same anterior half embryo 

 of about the 250-cell stage; in all the figures the embryo is viewed 

 from the dorsal side, but in Fig. 49 the focus is high and only 

 the ectoderm and neural plate cells of the dorsal surface are 

 shown; Fig. 50 is a median optical section showing chorda and 

 endoderm cells surrounded on the anterior side by ectoderm; 

 Fig. 51 represents the ectoderm of the ventral surface which is 

 visible at a deep focus. This half embryo is exactly like the 

 anterior half of a normal one in the formation of the neural plate, 

 the chorda plate, the general ectoderm and gastral endoderm, in the 

 overgrowth of the dorsal lip of the blastopore, even in the position, 

 shape and size of the individual cells, {cj. Figs. 9 and 10.) 



Finally in Fig. 52 there is represented an anterior half embryo 

 22 hours after the posterior cells were killed, and at a stage 

 when normal larvae of corresponding age have already under- 

 gone metamorphosis. The ectoderm has not yet inclosed the 

 embryo on the side next the injured cells, and this rarely happens 

 in anterior or posterior half embryos. The neural plate has not 

 rolled up nor invaginated to form a tube, though it is slightly 

 depressed along its median line; two sense spots are present though 

 there is no sense vesicle. The large rounded chorda cells are 

 irregularly scattered along the posterior border of the embryo, 

 where they project beyond the ectoderm; they never form a noto- 

 chord. There is no trace of yellow crescent substance nor of 

 muscle cells in these anterior larvae and no indication whatever of 

 a tail. They are, therefore, altogether unlike the normal larvae 

 and they afford complete and convincing evidence that the anterior 

 blastomeres of the ascidian egg are not totipotent but rather that 

 the development is a mosaic work. 



