296 ARTHUR WILLEY. 



The results, moreover, to which my friend Dr. Johan Hjort 

 of Christiania had come in his investigations as to the deve- 

 lopment of the hypophysis and ganglion in the buds of 

 Botryllus, determined me to study the same question in the 

 case of the metamorphosing larvae of Ciona intestinalis 

 and Clavelina lepadiformis more closely than I had at 

 first intended. Dr. Hjort had the kindness to show me his 

 preparations and to make me thoroughly acquainted with his 

 results, a preliminary account of which has appeared in the 

 'Zoologischer Anzeiger ' (No. 400, 1892) .^ 



Hjort^s results are of unusual interest, as they place the 

 contrast between the organogeny in the larva and in the bud 

 respectively of Botryllus in the clearest possible light. 



1. Closure of Neuroporus and Origin of Sense- 

 organs in Ascidia mentula. 



For the earliest stages of the nervous system after the fusion 

 of the medullary folds I examined the embryos of Ascidia 

 mentula, as they are much more transparent than those of 

 Ciona. 



Very soon after the commencement of the curvature of the 

 embryo within the follicle, the curvature being initiated and 

 necessitated by the outgrowth of the tail, the neuroporus, as 

 was correctly described by Kowalevsky (I, 21), closes. It 

 will be seen later that this primary closure of the neuroporus 

 in the Ascidians is only temporary, and does not occur in 

 Araphioxus ; while what may be called the secondary or final 

 closure occurs in both the Urochorda and the Cephalochorda. 



After the first closure of the neuroporus has taken place, 

 the nervous system of the Ascidian embryo consists of a 

 perfectly closed tube lying immediately below the epidermis, 

 and containing a lumen which is slightly dilated anteriorly, 

 the neurenteric canal having been obliterated at a somewhat 

 earlier stage (PL 18, fig. 1). In fig. 1 is represented an optical 

 sagittal section of an embryo of Ascidia mentula at the 

 stage in which the first trace of the sense-organs appears, in 

 ' la the same uuuiber a prelimiuary note on mj own results appeared. 



