ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG. 103 



lization of the ascidian egg is not unique, and that the whole movement is peculiar 

 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- 

 zing character. In 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 w r alls. 



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 he no doubt that in many eggs the localization which 

 begins before cleavage continues during that process. 



To a certain extent cleavage may he 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). The 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). The neural plate 

 arises on the anterior side of the egg 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 (A 63 , AH figs. XVII. XIX. 110, 117). At the next cleavage 

 these two portions are separated, the upper protoplasmic part becoming the neural 



