332 



«ine angenehme Pflicht meinem verehrten Lehrer Herrn Prof. Hert- 

 wig und Herrn Geheimrath Dohrn für alle die mir bewiesene 

 Freundlichkeit meinen besten Dank auszusprechen. 

 München, 29. Juni 1892. 



4. On the Development of the Hypophysis in the Asoidians. 



By Arthur Willey, B.Sc, London (Columbia College, New York). 



eingeg. 25. Juli 1892. 



Having been engaged for some time past in studying the deve- 

 lopment of several forms of Ascidians, I feel obliged to send in a pre- 

 liminary note on that portion of my researches which relates to the 

 so-called Hypophysis of the Ascidians, since my friend Dr. Johan 

 H j o r t informs me by letter that he has come to similar results in 

 the case of Distaila magnüarta to those to which I have arrived, and 

 it seemed desirable that our respective communications should appear 

 approximately at the same time. A mutual confirmation of this kind 

 in such a matter as that which forms the subject of this note , by two 

 independent observers working on very different genera of Ascidians is 

 satisfactory in the highest degree. 



The origin of the nervous system in the buds of Botryllus , as 

 determined by Hj ort who had acquainted me with his results, in- 

 creased my desire to control the work of van Beneden and Julin 

 (Le Système nerveux central des Ascidies. Arch, de Biologie. T.V. 1 884) 

 by personal observation. I have accordingly worked out the deve- 

 lopment of the Hypophysis in Ciona intestinalis and Clavielina lepadi- 

 f or mis. 



Clavelina was the form on which van Beneden and Julin ba- 

 sed their account. As a matter of fact the Hypophysis together with 

 the definitive ganglion arises in essentially the same way in both the 

 above mentioned forms and in a way fundamentally different from that 

 described by van Beneden and Julin. 



As stated by Kowalevsky, the Neuroporus of the Ascidian 

 embryo closes up at an early stage of development and the nervous 

 system then consists of a perfectly closed tube with a dilated anterior 

 extremity lying below the epidermis. 



Soon after the invagination of the stomodaeum and the subse- 

 quent perforation of the mouth the nerve-tube acquires secondarily an 

 opening into the stomodaeum, although I have never succeeded in 

 seeing this opening at such an early stage as that figured by Kowa- 

 levsky notwithstanding that I have looked for it in Ciona intestinalis^ 

 Ascidia mentula and Phallusia mammillata. 



