THE ORIGIN OF THE TEST-CELLS OF ASCIDIANS. 



T. 11. MORGAN, Ph.D. 



A WORD of explanation and apology seems necessary on add- 

 ing another account to the long list of descriptions of the origin 

 of the test-cells. While studying the embryology of Clavellina 

 in the spring of 1888, I became interested in the origin of the 

 test-cells, and the work was continued during the summer of 

 the same year. A preliminary note was written in October, '88, 

 and published in the Johns Hopkins University Circular, Vol. 

 VHL, No. 72. 



At the same time I obtained the paper of Van Beneden and 

 Julin in the Archives de Biologic, Tome, VL, 'Z^, in which I 

 found conclusions almost exactly similar to those to which I 

 had come. It seemed then unnecessary to publish a full account 

 of the work, and the figures and descriptions were laid aside. 



In the spring of '89, Dr. M. v. Davidoff published a new 

 account of the origin of the test-cells (Mittheilungen aus der Zool- 

 ogischen Station zu Neapel), in which he differs essentially from 

 Van Beneden and Julin. Thus the whole question became once 

 more unsettled by the conflicting accounts of Van Beneden and 

 Davidoff, and seemed worth working over again. 



During the summer of '89 I carefully prepared ovaries of 

 several Ascidians by the methods described in detail by David- 

 off, hoping in this way to meet him to some extent on his own 

 grounds, and to test the value of the new methods of prepara- 

 tion. During the winter of '89-90 this material was examined, 

 and gave not the slightest evidence of such an origin of the 

 test-cells as described by Davidoff. Here again I was irresist- 

 ably led to the same conclusions as those reached by Van 

 Beneden. 



As the results obtained by the methods described by Davidoff 

 differed in no essential point? from those obtained by other 

 methods, it seemed unnecessa,y to draw all the figures from 

 such preparations. Several genera were examined, including 



