Mosaic Development in Ascidian Eggs. 147 



deep yellow protoplasm which later enters into the muscle cells 

 of the tail of the larva; (2) the light yellow material which becomes 

 mesenchyme; (3) the light gray material which forms the chorda 

 and neural plate; (4) the slate gray substance which becomes 

 endoderm, and (5) the clear transparent protoplasm which gives 

 rise to the general ectoderm. All of these substances are recog- 

 nizable in the egg before the first cleavage and immediately after 

 that cleavage they all occupy their definitive positions in the egg, 

 the yellow protoplasm forming a yellow crescent around the pos- 

 terior side of the egg just dorsal to the equator, the light gray 

 substance forming a gray crescent around the anterior border of 

 the egg, the slate gray substance lying at the middle of the dorsal 

 hemisphere and between the two crescents, while the transparent 

 protoplasm is chiefly localized in the ventral hemisphere of the 

 egg. In these positions and from these substances the organs and 

 germinal layers specified arise. 



At the first cleavage of the egg all of these substances and areas 

 are equally divided, since this cleavage lies in the plane of bilateral 

 symmetry of the egg and future embryo. The second cleavage 

 plane is perpendicular to the first and separates the gray crescent 

 in front from the yellow crescent behind; the cells of the anterior 

 quadrants are therefore very unlike the posterior ones and the 

 two can always be distinguished at a glance. (Fig. i.) The third 

 cleavage is equatorial and separates four clear ventral cells from 

 four dorsal ones which contain the yellow and gray crescents and 

 the deep gray material. (Fig. 2.) The ectoplasm is now com- 

 pletely segregated in the four ventral cells but the other ooplasmic 

 substances are not as yet located in separate cells, though from the 

 time of the first cleavage onward their locations and boundaries 

 are perfectly sharp and distinct. 



At the fourth cleavage each of the eight cells divides, thus giving 

 rise to sixteen cells (Fig. 3) and at the fifth cleavage these are 

 increased to thirty-two. During the fifth cleavage the substance 

 of the gray crescent is segregated into four cells (A^-^, A**-*, Fig. 4)^ 

 at the anterior border of the egg, while the yellow crescent comes 



'The system of cell nomenclature employed in this paper is similar to that used by Castle ('96) and 

 is fully explained in my work on the cell-lineage ('05'); in brief A and a designate cells of the anterior 

 half of the egg, B and b those of the posterior half, the capitals being used for cells of the vegetal (dorsal) 

 hemisphere, the lower case for those of the animal (ventral) hemisphere. Corresponding cells of the 

 right and left sides receive the same designation, except that those of the right side are underscored. 



