ASCIDIACEA FIXATION AND METAMORPHOSIS. 



379 



contact and fuse, giving rise to the internal longitudinal bars (Ir) 

 of the branchial region. 



At an early stage, the first rudiment of the pericoronal circle 

 (peripharyngeal bauds, ciliated arch, Fig. 174 A,fb), as well as that of 

 the coronal circle, which lies in front of it and is beset with tentacles, 

 can be seen. The arrangement of the tentacles when they first appear 

 is bilateral. 



The development of the genital organs has still to be described. 

 In the compound Ascidians, the individual that develops from the egg 

 has no genital organs, but multiplies exclusively through budding. 

 This is also the case with the social Ascidians (Ganin). In studying 

 the development of the genital organs we are consequently restricted 

 to the simple Ascidia if we wish to study their origin in a sexually 



<& 



Fig. 1 75. — A, dorsal aspect of the intestinal coi] in the bud of Perophora Listeri with 

 the rudiment of the genital organs; B, somewhat older genital vesicle (after van 

 Benedex and Julis). g, genital vesicle; gs, genital strand; dr, digestive gland; 

 oe, oesophagus; m, stomach; i, intestine. 



produced embryo, otherwise we must trace their rise in the asexually 

 produced buds of other Ascidians. The course of development in 

 these two cases is, however, so uniform that we may take as our 

 example the buds of Perophora which were examined with special 

 reference to this point by van Beneden (No. 10). 



The Ascidians are hermaphrodite. The male and female genital 

 rudiments, however, are derived from a common rudiment which is 

 always unpaired and lies on the intestinal loop medianly and dorsally 

 (Figs. 174 B, gs', 238 E, g). This is found at the point at which the 

 efferent duct of the digestive gland first branches (Fig. 175 A). It 

 consists of an accumulation of cells indistinguishable from ordinary 



