458 



Mr. A. G. Huntsman. 



[Mar. 3, 



the oral region of an Ascidian, in which all parts, except those of neural 

 origin, are represented by dotted lines. 



The three genera that have been the subject of this investigation belong 

 to three different families, and one of them (Clavelina) is almost at the 

 opposite extreme of the Ascidian series from the other two. I presume, 

 therefore, to think that this origin of the mouth is characteristic of all or 

 of most Ascidians, and that any exceptions will show only modifications of 

 this. It affords a satisfactory explanation of the opening of the neural gland 

 or hypophysis into the oral siphon. 



As to the general significance of this developmental process, I think that 

 comparisons between this process and that to be found in Cephalochordates 

 and in Vertebrates should be used with much caution. I believe that the 

 majority of the homologous organs of the three groups have been evolved 

 independently, and are so remarkably alike because they have been derived 

 from an original similar condition in the groups, this condition being the 

 constitution of the germ plasm rather than the presence of these organs. 



With this reservation, I make the following homologies (most of which 

 have been made previously), with the organs in order from behind forwards 

 and downwards. 



Tunicate. 



Cephalochordate. 



Vertebrate. 



Ganglion 



Sensory vesicle 



Neural gland and duct 



Neural part of oral siphon... 



Ant. end of nerve cord. . . 

 Eye-spot 



Olfactory pit 



Main part of brain. 



Pineal eye and retina of lateral eyes. 



Auditory vesicles. 



Hypophysis and nasal cavities. 



Stomodseum. 



The order is faulty only in the case of the auditory vesicles of Vertebrates. 

 They occur laterally and behind the eyes. 



The three groups form a series as to the connection of the different organs 

 with each other and as to the times at which they invaginate from the 

 ectoderm. In the Tunicate they are all connected together and invaginate 

 very early to form the neural tube. In the Cephalochordate the first two and 

 the fourth (the third is absent) are connected. The first two form part of 

 the neural tube, whereas the fourth invaginates later but at the neuropore. 

 The fifth (stomodseum) is separate from the others. In the Vertebrate only 

 the first two are connected together and form part of the neural tube. The 

 others are more or less separate and invaginate later. 



These structures are all median and unpaired, except the three sense organs 

 of the Vertebrate, which are usually paired. 



