ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG. 103 
lization of the ascidian egg is not unique, and that the whole movement is péculiar 
only because of its extent and the definite manner in which it is directed. 
The movements which take place during cleavage are in part merely the 
general movements which accompany cell division and in part they are of a locali- 
zimg charaeter. Іп the former class are the vortical movements which probably 
cause the separtion of the chromosomes and the division of the cell body (Conklin, 
1902); in the latter are such movements as that which occurs at the close of the 
first cleavage by which the clear protoplasm is carried from a central position into 
the upper hemisphere of the egg. After the cleavage has begun the localizations 
due to movement are strictly limited to the individual cells, no movements of a 
localizing character occurring through cell walls. 
b. Cell Division as a Factor of Localization. 
This brings us to the much discussed question of the role of cell division in 
development, and more particularly of the influence of cell division on phenomena of 
localization. There can be no doubt that in many eggs the localization which 
begins before cleavage continues during that process. 
To a certain extent cleavage may be regarded as a localizing factor, but its 
importance in this respect is certainly far less than that of the active movements 
just described. Inasmuch as localizations may take place in the absence of cleav- 
age or before it begins, and since many cleavages are non-differential it is evident 
that there is no close nor necessary connection between the two. Furthermore 
the cleavage planes do not always coincide with the lines of localization ; this is 
shown especially well in the ascidian, where the localization in the unsegmented 
egg is particularly distinct. Thus the cleavage planes do not follow closely the 
boundaries of the crescent; the first and second cleavage planes are placed symme- 
trically with reference to the crescent, but they do not coincide with any of its 
boundary lines. — The third cleavage plane lies above the upper border of the 
crescent when first formed; later the crescent extends up to the equatorial plane 
so that the cleavage plane and the upper boundary of the crescent coincide (fig. 31, 
32). Тһе fourth cleavage cuts off the median posterior crescent cells from the 
lateral ones, but leaves an area of yolk in both of these cells (fig. 37). In the 
median posterior cells this is a small wedge-shaped mass of yolk which is later 
covered and obscured by the yellow crescent substance (fig. 39). Тһе neural plate 
arises on the anterior side of the есе from cells which lie both above and below the 
equator, or plane of the third cleavage; these neural plate cells are rich in pro- 
toplasm, and correspondingly the area from which they arise is richly protoplasmic. 
The third cleavage cuts right through this protoplasmic area leaving a portion of 
it above and a part below the equator. In the 8-cell and 16-cell stages the anterior 
dorsal cells contain both neural-plate and chorda substance; the portion of each of 
these cells turned toward the equator is protoplasmic, that turned toward the vegetal 
pole yolk-laden (А, A*5, figs. ХУП, XIX, 116, 117). At the next cleavage 
these two portions are separated, the upper protoplasmie part becoming the neural 
