316 ATITHUR WILLEY. 



oral portion in the Vertebrates is the pituitary body, and in 

 the Ascidians the proximal portion of the hypophysis, in- 

 cluding the dorsal tubercle. 



III. 



On the Position of the Mouth in the Larvae of the Ascidians 

 and Amphioxus, and its Relation to the Neuroporus. 



(With Plate 20.) 



In the first of these " Studies " I have quoted a sentence of 

 Kupffer, in which he says that the pronounced dorsal position 

 of the raouth in the Ascidian tadpole is occasioned by the pre- 

 sence of what I have called the praeoral lobe, which contains 

 the anterior body-cavity. But in Balanoglossus, where an 

 homologous anterior body-cavity or proboscis-cavity is present, 

 the mouth is ventral. So that, according to this point of view, 

 what causes the mouth to be dorsal in one case causes it to be 

 ventral in another. 



The way out of this dilemma is found as soon as the fact is 

 recognised that the anterior body-cavity has nothing to do per 

 se with the position of the mouth, and that at least in the 

 groups of the Protochordata (Cephalodiscus, Balanoglossus, 

 Tunicata, Amphioxus) the dorsal or ventral position of the 

 mouth does not affect the homology of organs which lie in 

 front of it, for the reason that there is every evidence to show 

 that the anterior body-cavity in all these forms is not a truly 

 median structure, but has, either actually or virtually, a bi- 

 lateral origin. 



In Balanoglossus, as shown by Bateson, the anterior body- 

 cavity arises at first as a perfectly median archenteric pouch. 

 It becomes, however, in the course of the development, incom- 

 pletely divided into two by the formation of a mesenchymatous 

 septum, in which lie the so-called heart, notochord, and pro- 

 boscis-gland. But perhaps the strongest evidence of the essential 

 bilaterality of the proboscis-cavity of Balanoglossus is, that 

 while in most forms there is only one proboscis-pore, namely. 



