10 ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG. 



a slight eccentricity of the nucleus, it is often difficult to observe it after the dis- 

 appearance of the yolk matrix. No other axial differentiations of the egg are 

 recognizable until after the fertilization. 



2. Test Cells and Chorion. 



In the fully formed ovarian eggs the test cells lie imbedded in a peripheral 

 layer of clear protoplasm ; this layer stains intensely with plasma stains, and in the 

 living eggs of Cynthia contains yellow pigment granules. In Cynthia the test cells 

 are distributed singly and pretty uniformly in this peripheral layer (fig. 61), and 

 the same is true of Ciona at an early stage in the formation of the ovocyte (figs. 168, 

 169), but in the fully formed ovarian egg of Ciona the test cells are found in little 

 masses or "nests" of from three to six or eight cells each (fig. 170). A similar 

 grouping of the test cells has been described by Morgan (1890) in an unidentified 

 species of Clavellina. These cells are much smaller and more numerous than the 

 test cells of Cynthia, and are evidently formed by division of the original test cells. 



The test cells of Cynthia become quite large and contain yolk spherules, though 

 they do not stain as densely as the yolk of the egg; in Ciona the test cells are very 

 much smaller and do not contain these spherules. About the time that the ovarian 

 eggs escape from the ovary the test cells are extruded from this peripheral layer of 

 protoplasm, and the outlines of the egg, which up to this time have been irregular. 

 become more nearly spherical. It is probable that the expulsion of the test cells 

 and the assumption of the regular spherical form by the egg have a common cause 

 in the increase of surface tension at this time. 



At the time of the extrusion of the test cells I have observed in the ovarian 

 eggs of Ciona a faintly-staining, homogeneous layer which lies inside the outer 

 follicle cells and outside of the egg. The test cells lie on the inner border of this 

 homogeneous layer; from its general appearance it is highly probable that the sub- 

 stance of which it is composed is extruded from the egg along with the test cells. 

 This homogeneous material does not long persist as such but soon disappears and 

 probably goes to form the chorion. At this time the egg undergoes considerable 

 shrinkage in size, a distinct perivitelline space being formed, and the egg becoming 

 regularly spherical [cf. figs. 171 and 172). It is evident that this is clue to the 

 escape of fluid from the egg, probably the homogeneous substance described above. 



In this connection a word or two as to the significance of the test cells may be 

 permissible. The fact that in Cynthia they contain yolk and grow to a considera- 

 ble size, and that spermatozoa not infrequently enter them (figs. 80 and 85 sn.), may 

 be taken as evidence that these cells are rudimentan' eggs ; a view which is held by 

 Floderus (1896). Bancroft (1899) and others. 



!. Structure of Fully Formed Ovocyte. 

 When first laid the living eggs of Cynthia are, exclusive of the egg envelopes, 

 about 150 /< in diameter; those of Ciona are about the same size, but in Molgula 

 they are much smaller, being about 100 /< in diameter. The very large germinal 



