Mosaic Dcvelopmcj-it in Ascidian Eggs. 189 



notochord when once they have escaped and have become free, a 

 certain amount ot compression being necessary to bring about the 

 characteristic interdigitation which leads to the formation of a rod- 

 shaped notochord.^ 



The posterior quadrants can be distinguished in all eggs at all 

 stages by the presence of the yellow crescent substance or cells. 

 In early stages, as I have shown, these crescent cells are normal 

 in position and character; in later stages the yellow cells fill the 

 whole interior of the embrv'o. When once these cells have been 

 inclosed by the ectoderm I have been unable to recognize any 

 constancy in their position and arrangement. As in the posterior 

 half embryos, a tail is never formed in these posterior quarter 

 embryos and the muscle cells are never elongated, both these 

 features evidently depending upon the presence of a notochord. 

 The caudal endoderm cells are found in most, if not all, of these 

 posterior quarter embryos as a single row^ of volk laden cells which 

 lie along the first cleavage plane (Figs. 65-68), the position which 

 they normally occupy. 



These quarter embryos show in the most unmistakable manner 

 that the development is strictly partial, and that an individual 

 blastomere never gives rise to parts ivhich it would not produce in 

 the entire embryo. Among the hundreds of quarter embryos which 

 I have studied both in the living condition and as stained and 

 mounted preparations I have never seen a single one ivhich even 

 remotely resembled a normal larva. 



6. Eighth or Sixteenth Embryos (Figs. Jl—j6). 



When eggs are spurted or shaken in the 8-cell and i6-cell stages 

 a great variety of abnormal forms are produced, a few of which 

 are shown in Figs. 71 and 73-76. Without exception, however, 

 the same principles apply here as in the case of half and quarter 

 embryos, viz: a given blastomere or group of blastomeres produces 

 only those parts of an embry^o or larva which would develop from 

 it under normal conditions. Fig. 71 represents an embryo 

 derived from the dorsal anterior eighth of an egg (the cell A^-^^j 

 14 hours after the injury. Normally this eighth gives rise to 

 neural plate, chorda, gastral endoderm, and a small amount of 



'Chabry, however, figures (his Fig. i8) a partial embryo with a rod-shaped notochord lying outside 

 the embrj'o in the perivitelline space. 



