1913.] 



On the Origin of the Ascidian Mouth. 



457 



In the latter form the extent of the neural tube material is more easily seen 

 than in either of the other forms, since the cells of the neural tube contain 

 much less yolk than those of the ectoderm and endoderm. 



There is great difficulty in tracing the further fate of the neural, ectodermal 

 and endodermal cells respectively of the oral region. I believe, however, 

 that the following conclusions are warranted. 



At the time when the ectodermal cells begin to invaginate at the position 

 of the old neuropore, test has begun to form on the surface of the ectoderm. 

 The cells invaginated have test on what are or will be their free surfaces. 

 Doubtless they continue the formation of test. If so, the ectodermal part of 

 the adult will consist of the test-bearing epithelium of the siphon, the 

 so-called " reflected tunic." The hypophysial canal of the late embryo and 

 larva opens near the margin of the endoderm (fig. lc). If this position is 

 retained, the junction of the neural and endodermal regions of the adult will 

 be immediately behind the dorsal tubercle. In this position we have the 

 peripharyngeal ciliated bands. These connect ventrally with the right and 

 left lips of the endostyle. The peripharyngeal bands have every appearance 

 of being a dorsal continuation of the endostyle, split into right and left 

 halves by the sinking down of the neural tube. The subsequent fusion of 

 the latter with the margins of the opening thus formed in the pharynx 

 causes the two halves of the split endostyle to form a circular band at the 

 anterior extremity of the pharynx. 



The parts derived from the three embryonic layers will then be, 



Ectodermal Test-bearing epithelium or " reflected tunic." 



Neural Pretentacular zone. 



Tentacular ridge and tentacles. 



Prebranchial zone and dorsal tubercle. 

 Endodermal ... Peripharyngeal bands and parts behind. 

 This is illustrated in fig. 2, a diagrammatic longitudinal section through 



Fig. 2. — Diagrammatic sagittal section through oral region of adult Ascidian. dt, dorsal 

 tubercle ; en, endostyle ; gl, neural gland ; gn, ganglion ; pb, peripharyngeal band ; 

 tn, tentacle ; ts, test. 



ts 



pb' 



VOL. LXXXVI. — B. 



2 L 



