36 ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG. 



to a position just below the equator on the posterior side where it forms a yellow 



crescent. At the first cleavage this crescent is divided in the middle into right and 

 left halves: at the second cleavage it passes into the two posterior cells of the 4-cell 

 stage; at. the third cleavage it goes into the two posterior vegetal cells of the 8-cell 

 stage. In two subsequent divisions the yellow protoplasm is separated from the 

 yolk with which it is associated and thereafter forms a crescent of yellow cells which 

 surrounds the posterior side of the egg just below the equator (figs. 37, 39, 41, 42). 

 At all stages of development this crescent or. the cells which arise from it, lies in 

 the posterior half of the vegetal hemisphere, and the yellow cells are never sepa- 

 rated from the mid-dorsal line by more than a single row of yolk cells (figs. 44-48). 

 On the other hand these yellow cells are separated from the mid-ventral line by an 

 ever increasing number of clear protoplasmic cells (figs. 43, 45, 122, 129, 137, et 

 seq.). The single row of yolk cells mentioned above as lying between the yellow 

 cells and the dorsal mid-line invaginates during: gastrulation and aives rise to the 

 ventral cord of endoderm in the tail of the larva, while the yellow cells, which are 

 also invaginated, trive rise to the mesoderm. A study of this yellow crescent and 

 of the cells which develop from it shows conclusively that it always lies on the pos- 

 terior border of the yolk-rich or dorsal hemisphere, that at the 16-cell and 32-cell 

 stages, it is separated from cells which give rise to the ventral endoderm, and that 

 it is invaginated with the endoderm and forms the muscle cells and mesenchyme of 

 the tadpole. 



(3) Wholly similar results as to the orientation of the egg and embryo follow 

 from a study of the lineage of all the other cells of the embryo. I believe that I 

 have seen every division of every cell up to the 21 8-cell stage, and in the critical 

 period between the 32-cell and 7(>-cell stages I have seen these divisions in hundreds 

 of cases. The evidence from this source as to the orientation cannot here be pre- 

 sented in detail but must be deferred to that portion of this paper which deals 

 particularly with the cell-lineage ; however, it can be said that in not a single inst- 

 ance have I found any evidence against the orientation according to Van Beneden 

 and Julin, while every observation which I have made on the cell-lineage speaks in 

 favor of that orientation. 



(4) Finally a most direct and convincing evidence in favor of this system 

 of orientation is found in the position of the polar bodies throughout develop- 

 ment. In preparations of the eggs of the three genera of ascidians which I have 

 studied, the polar bodies are easily distinguishable from the test cells by their 

 deeper stain ; in Ciona they are also larger than the test cells. In the last named 

 genus I have seen the polar bodies attached to the egg or imbedded in it at every 

 stage from the unsegmented egg to the gastrula (Plates XI and XII). In every 

 single instance they have been found at a point on the ectodermal (ventral) hemis- 

 phere which a study of the cell-lineage shows to correspond to the animal pole of 

 the unsegmented egg. I have not observed the polar bodies in every egg of Cynthia 

 which I have studied or drawn, possibly because they do not in this genus remain 

 attached to the egg so persistently as in Ciona. but wherever I have been able to 



