ORGANIZATION AM) CELL-LINEAGE OF ASCIDIAN EGG. 9 



could puss in, after the manner recommended l>\ Chabry. In most cases I have 

 found that the presence of the egg envelope's (iocs not seriously interfere with clear 

 seeing, possibly owing to the fact that in the study of preparations 1 have used an 



immersion lens in which the depth of locus is relatively slight. In late stages, 

 however, the test cells are sometimes confusing, and in the case of Ciona 1 found 

 that these, together with the other envelopes, could he removed by simply rolling 

 the eggs under the cover glass. In Cynthia the envelopes may sometimes he 

 removed in the same way, though not so easily as in Ciona. 



I. THE OVARIAN EGG. 



Much has been written on the egg envelopes and ovarian eggs of ascidians 

 and I shall not here go over that ground in any detail. But in searching for the 

 earliest different iations of the egg substance it is necessary to go back to the 

 ovarian egg, and in so doing I have found some structures the real significance of 

 which has not hitherto been appreciated. 



1. Developynent of the Ovocyte. 



In a young ovocyte the cytoplasm stains uniformly and there is no trace of 

 yolk or of test cells. Close around the nucleus is a granular mass which is deeply 

 colored by plasma stains, the yolk nucleus or "yolk matrix" of Crampton (1899). 

 As the egg grows, small spherules of yolk begin to appear in the vicinity of the 

 yolk matrix, and this yolk gradually fills the central portion of the egg surrounding 

 the nucleus, while the cytoplasm, which is free from yolk, occupies a peripheral 

 position. Some of the follicle cells which surround the egg at this stage then 

 invade the egg, thus forming the " test cells" which are located chiefly in the 

 peripheral layer of cytoplasm. My observations as to the origin of these " test 

 cells " agree with some of the most careful work, both ancient and modern, which 

 has been done on this subject (Kowalevsky, 18G6, 1871; Seeliger, 1882; Van 

 Beneden and Julin, 1886; Morgan, 1890; Floderus, 189G ; Bancroft, 1899). 



The earliest appearance of polarity is found in the location of the yolk matrix 

 on one side of the nucleus and in a slight eccentricity of the latter. I consider it 

 very probable that the yolk matrix is derived from the attraction sphere of the last 

 ovogonic mitosis, and that the chief axis of the egg represents the cell axis which 

 passes through the centrosome and nucleus, and which, as I have previously shown 

 (Conklin, 1902), is preserved in every cell throughout the cleavage of the egg and 

 probably also in all later cell divisions. If this be true, the polarity of the egg is a 

 differentiation which is carried over from generation to generation, and as this chief 

 axis of the egg is identical with the gastrular axis, and bears a constant relationship 

 to the principal axes of the embryo and adult, it will be seen that at least one 

 important differentiation of an animal is predetermined (not predelineated) at all 

 stages. Although this chief axis of the egg is usually recognizable at all stages by 



Z JOURX. A. N. S. PHILA., VOL. XIII. 



