200 PROCEEDINGS OF THE AMERICAN ACADEMY. 



VIII. 



CONTRIBUTIONS FROM THE ZOOLOGICAL LABORATORY OF 



THE MUSEUxM OF COMPARATIVE ZOOLOGY, UNDER 



THE DIRECTION OF E. L. MARK, XLII. 



ON THE CELL LINEAGE OF THE ASCIDIAN EGG. 



A PRELIMINARY NOTICE. 



By W. E. Castle. 



Communicated by E. L. Mark, June 14, 1894. 



The following paper is based on an uncompleted study of the early 

 stages in the development of C'iona intest'malis, the material for which 

 I was enabled to collect at the Newport Marine Laboratory in the 

 summer of 1893, through the kindness of Mr. Alexander Agassiz. 



Tunicate embryology has been studied from the standpoint of cell 

 lineage to a greater or less extent, by Van Beneden et Julin ('84), 

 Seeliger ('85), and Chabry ('87). The first named authors have 

 followed the segmentation of the egg of Clavelina Rissoana as far as 

 the 32-cell stage in a manner that admits of no question as to its 

 accuracy. Every cell division is evidenced by karyokinetic figures 

 that prove beyond doubt the actual genetic relationship. 



The cell lineage of a single additional stage is given, one of forty- 

 four cells. This stage must have been derived from the preceding by 

 twelve cell divisions, six in each of the equivalent halves of the egg. 

 Of these six, three are clearly indicated by spindles in the 32-cell 

 stage. The other three were apparently determined by conjecture, 

 and it is my purpose to show that by yielding themselves to conjec- 

 ture in so small a matter as these three cell divisions, the eminent 

 authors fell into an error which invalidates the most important con- 

 clusions of their otherwise excellent work. For in correlating the 44- 

 cell stage with the 32-cell stage they have changed the orientation, so 

 that they have identified the dorsal side of one with the ventral side 

 of the other, the endodermal half of one with the ectodermal half of 

 the other. Their orientation of all the stages prior to the A^-cell stage 



