Mosaic Development in Ascidimi Eggs. 219 



derm are frequently present in their normal positions and num- 

 bers. A tail is never formed in these anterior embryos and they 

 bear no resemblance whatever to a normal larva. 



11. Posterior Half Embryos. Embryos derived from the two 

 posterior quadrants have no trace of notochord or of chorda cells, 

 neural plate, sense vesicle, sense spots, or gastral endoderm; they 

 contain a mass of muscle and mesenchyme cells and a double row 

 of caudal endoderm cells, as in the normal embryo. There is no 

 indication of a tail or head, the embryo remaining rounded in form 

 as long as it lives. 



12. Three-Quarter Embryos. Embr}^os derived from three of 

 the first four blastomeres are more nearly perfect than are half 

 embryos, but they always show defects corresponding to the 

 missing blastomere. If an anterior blastomere is killed, the 

 neural plate and sense vesicle of the resulting larva are atypical 

 and the notochord lacks the normal number of cells; if a posterior 

 cell is killed, the muscle and mesenchyme cells are lacking along 

 one side of the tail. 



13. One-Quarter Embryos; Two-Quarter Diagonal Embryos. 

 Embryos derived from any one quadrant or from two diagonal 

 quadrants of the egg are always very defective. They never have 

 a notochord, though if they come from anterior quadrants they 

 may give rise to scattered chorda cells in the perivitelline space; 

 there is never a sense vesicle, though if they are from an anterior 

 quadrant a neural plate and sense spots are present. The poste- 

 rior quadrants always contain muscle, mesenchyme and caudal 

 endoderm cells, but never a trace of notochord, neural plate nor 

 sense spots. The embryos are always rounded, there being no 

 distinction of head and tail, and in no respect do they resemble 

 normal larvae. 



14. Eighth and Sixteenth Embryos. When blastomeres are 

 injured in the 8-cell or i6-cell stages a great variety of abnormal 

 forms are produced. Ventral blastomeres give rise only to 

 rounded masses of ectoderm cells in which there is no trace of 

 endoderm or mesoderm; posterior dorsal cells give rise only to 

 muscle, mesenchyme, and caudal endoderm; anterior dorsal cells 

 to neural plate, chorda, and gastral endoderm. 



15. Anterior and Posterior Half Gastrulcs. When cup-shaped 

 gastrulae of the stage shown in Fig. 8 are cut in two transversely 

 so as to leave all of the yellow cells in one half and all of the chorda 



