380 



TUNICATA. 



mesenchyme-eells^ and of a uniserial cell-strand which runs forward 

 and reaches the atrial aperture (genital strand, Fig. 175 B, gs). In 



the next stage, a cavity appears 



B 



in the cell-accumulation (Fig. 

 175 A). This vesicular rudi- 

 ment of the genital glands is 

 soon divided into two diverticula 

 by a transverse constriction (Fig. 

 1 75.#), one of these representing 

 the rudiment of the male gland 

 and the other that of the female 

 gland and its efferent duct. Of 

 these two diverticula, the inner 

 one, that which lies ventrally, 

 develops first and becomes flask- 

 shaped. Its dilated terminal 

 swelling (Fig. 176 A, h) becomes 

 the testis, and in this an ex- 

 ternal flat epithelial layer can 

 soon be distinguished from an 

 inner layer of spermatogonia. 

 The narrowed efferent duct (vas 

 deferens, vd) opens at first into 

 the female genital vesicle. In 

 this latter also considerable 

 elongation can be noticed (Fig. 

 176). While, near the blind 

 end, the germ-epithelium (k) 

 becomes differentiated, the re- 

 maining elongated part of the 

 rudiment becomes the oviduct 

 {<><]). At the time when this 

 oviduct elongates, the genital 

 strand ((/) which functions as 

 gubernaculum becomes corre- 

 spondingly shortened. It is 

 probable that the oviduct 

 develops to some extent at the 

 expense of the latter. In 

 this way, the end of the oviduct approaches the atrial wall more and 

 more till, after the complete disappearance of the genital strand, it 



Fig. 176. — Later stages in the development 

 of' the genital organs in the bud of Pero- 

 phora Listeri (after van Beneden and 

 Julin). (j, genital strand ; //, rudiment 

 of the testis ; h, germ-layer of the ovary; 

 od, oviduct ; i<v, rudiment of the ovary ; 

 vd, \ .is deferens. 



