Mosaic Development tn Ascidian Eggs. 21/ 



2. During maturation and fertilization the yellow substance 

 flows rapidly to the vegetal pole where it forms a superficial layer 

 or cap; the clear substance escapes from the germinal vesicle and 

 flows toward the vegetal pole where it forms a stratum above the 

 yellow cap; the gray substance occupies the animal half of the egg. 

 The localization at this stage is polar-radial. 



3. The sperm nucleus which lies in the center of the yellow 

 cap moves to the posterior pole of the egg and the yellow and clear 

 substances move with it The yellow material here forms a 

 crescent which lies with its center at the posterior pole and its 

 arms extending forward on each side about halfway around the 

 egg; the clear substance forms a band just above and internal to 

 the crescent; the gray substance occupies the remainder of the egg. 

 At this stage the localization is bilateral. 



4. The first cleavage furrow appears in the plane of bilateral 

 symmetry and divides each of the ooplasmic substances equally. 

 At the close of this cleavage the clear substance occupies the ani- 

 mal (ventral) half of the egg; the gray substance lies at the middle 

 of the vegetal (dorsal) pole while around the posterior border of the 

 dorsal hemisphere is the yellow crescent and around its anterior 

 border is a light gray crescent. This is the definitive localization 

 of these substances, and in these positions the clear material gives 

 rise to ectoderm, the gray to endoderm, the yellow crescent to 

 muscles and mesenchyme, and the gray crescent to chorda and 

 neural plate. 



5. The second cleavage is transverse to the antero-posterior 

 axis and separates the gray crescent from the yellow; the third 

 cleavage separates the clear protoplasm of the ventral hemisphere 

 from the various substances of the dorsal hemisphere. 



II. Experiments. 



6. Individual blastomeres were injured by spurting or shaking 

 the eggs in the 2, 4, 8, or i6-cell stages. The surviving blastomeres 

 were then studied both in the living condition and after being 

 stained and mounted. 



7. Cleavage. Isolated blastomeres always segment as if they 

 still formed part of the whole, except that the direction of some of 

 the cleavages is slightly altered so that the resulting cell mass is 

 more nearly spherical than in the normal egg. These alterations 



