96 WILLIAM BATESON. 



no insistance is placed on the following suggestion^ the plausi- 

 bility of it is such that it cannot be omitted. On a previous 

 occasion I have called attention to the fact that the pore which 

 in Amphioxus leads into the left anterior body cavity is ob- 

 viously homologous with the proboscis pore of BalanoglossuSj 

 which leads from the left horn of the anterior body cavity. In 

 some species of Balanoglossus the opening of this pore is placed 

 medianlyj though opening into the left horn. Now, supposing 

 the prseoral lobe to atrophy, as in an Ascidian, so that the 

 neural pore came to open into the buccal cavity, as occurs in 

 these forms, it is clear that any pore placed dorsally between 

 the neural pore and the mouth will then be directed ventrally, 

 and open into the pharynx below the end of the nervous 

 system. This is precisely the position occupied by the ciliated 

 pit of an Ascidian, which leads into the gland described by 

 Julin ('Arch, de Biol.,' 59). Hence with this pore and gland 

 of an Ascidian the proboscis pore and gland of Balanoglossus 

 may be compared. Next, supposing the end of the nervous 

 system to dilate and form a brain which bends up by a cranial 

 flexure it follows that on the atrophy of the proboscis (or rather 

 before the proboscis was formed, this being peculiar to En- 

 teropneusta) this pore will lie in the dorsal wall of the stomo- 

 dseum, i. e. in the position of the pituitary body. More than 

 this, any gland attached, as is the proboscis gland, to the end 

 of the notochord, will, when this is flexed by the cranial 

 flexure, be bent backwards with it to the place where its end 

 comes to lie, i. e. above the pituitary involution. In this way 

 the double structure of the pituitary body becomes intelligible. 

 If these views are correct the pituitary body and its pore is to 

 be regarded as the rudiment of a primitive excretory organ, 

 which originally opened dorsally. 



I have elsewhere shown the prima facie resemblance of the 

 anterior body cavity with its pore in Amphioxus to that of 

 Balanoglossus, which in the Tornaria development is formed 

 from the water-vessel (Spengel). This water-vessel is precisely 

 similar to that of Echinoderms, being otherwise without parallel 

 among animals. 



