26 ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG. 



separated than in the preceding figure, though they still lie nearer one pole of the 

 egg than the other. In this case also there can be little doubt that the more richly 

 protoplasmic pole corresponds to the posterior pole of normal eggs while the yolk- 

 laden pole corresponds to the anterior one. 



Other dispermic eggs arc shown in text figures III-VI, and here also one hemi- 

 sphere of the egg contains more protoplasm than the other, and may probably be 

 identified with the posterior pole. In still other eggs, especially those in which the 

 cleavage spindles are fully formed, the spindle and protoplasmic fields may lie on 

 opposite sides of the egg [cf. text fig. V). In these cases neither pole can be cer- 

 tainly identified as anterior or posterior. In normal eggs the cleavage spindle 

 always stands at right angles to the chief axis of the egg and to the plane of the 

 first cleavage; in dispermic eggs the spindles are frequently not at right angles to 

 the egg axis and if. as 1 believe, the plane between the two protoplasmic areas rep- 

 resents the normally median plane, they are more frequently parallel with this 

 plane than perpendicular to it. 



The phenomena of dispermy demonstrate that the point of entrance of the 

 spermatozoon is not predetermined but that spermatozoa may enter at different 

 points on the vegetal hemisphere; they also render probable the view that the 

 plane of bilateral symmetry is not first established by the accidental path of the 

 spermatozoon within the egg, but that this plane is structurally present before ferti- 

 lization. This problem will be more fully discussed in the general part of this paper 

 (Chap. VII). 



III. ORIENTATION OF EGG AND EMBRYO. 



As a preparation to the study of the cell-lineage and later development of the 

 ascidian vixix it is necessary to consider at once the orientation of the egg and early 

 cleavage stages. This is the more necessary since the utmost possible diversity of 

 opinion has been expressed with regard to this matter. 



1. Van Beneden and Jul Ms System of Orientation. 

 Van Beneden and Julin (1884) were the first to undertake to relate the 

 early stages of development of the ascidian egg to the later stages. Their work 

 was in fact one of the earliest and most admirable contributions to the subject 

 of cell-lineage. They followed the cleavage, cell by cell, as far as the 44-cell stage 

 and pointed out what they supposed to be the relations of each of these cells to the 

 germ Layers. They determined the relations of the axes of the egg and early cleav- 

 age stages to those of the gastrula and larva and, for the first time in the history 

 of embryology, established the fact that the principal axes of the larva may be 

 identified in the nnseginented egg. The evidence's upon which they based their 

 conclusions as to the axial relations of egg and embryo and as to the fate of the 

 cleavage cells are not fully stated in their brief paper of only fifteen pages; but 

 their statements of fact are perfectly clear and explicit. In brief these are as 

 follows : 



