THE ABBREVIATED DEVELOPMENT OF THE MOLGULIDAE. 381 



mines into contact with it. At the same time, the vas deferens has 

 been gradually split oft' from the oviduct so that finally the two- 

 canals enter the cloaca separately. The male and female genital 

 rudiments have thus become altogether distinct. The further de- 

 velopment of the genital glands manifests itself chiefly in the con- 

 tinuous formation of lobes. In the epithelium lining the ovary, the 

 eggs can soon lie distinguished from the surrounding undifferentiated 

 cells, which form the follicular epithelium. As the eggs increase in 

 size, they shift, enveloped in their respective follicles, into the 

 surrounding stroma, so that, finally, they are only connected, like- 

 the grapes on a bunch, by means of the thin efferent ducts of the 

 follicles with the ovarian epithelium. 



The urinary vesicles (Fig. 174 B, rs) form in the same way as the 

 Hist rudiment of the genital organs as accumulations of mesenchyme- 

 cells in which a cavity appears, containing at first only serous fluid,, 

 but. later, urinary concretions (van Beneden and Julin, No. 9). 



G-. The Abbreviated Development of the Molgulidae. 



The development of those forms the eggs of which up to the time 

 when the tailed larva hatches remain within the body of the mother 

 (Ascidiae compositae, Cynthia, Lithonephria) is, in many ways, some- 

 what modified and abbreviated. < hie of the Molgulidae which lays 

 its eggs affords us, however, curiously enough, the most marked 

 example of abbreviation of development. The caudate larval stage, 

 in this case, is altogether omitted (Lacaze-Duthiees, No. 36 ; and 

 Kupffer, No. 35) ; even the chorda does not appear to develop. In 

 other respects, the ontogeny of this form, so far as it is known, doe* 

 not seem essentially to differ from that of other Ascidians. After the 

 cleavage of the very opaque egg has taken place, the thick-walled 

 primary enteric vesicle can be recognised inside the embryo, and near 

 it an accumulation of large cells filled with reserve nutritive material. 

 This accumulation can be seen for a long time, as development pro- 

 ceeds, lying near the posterior end of the body (Fig. 177, r). It may 

 perhaps be regarded as the equivalent of the suppressed larval tail and 

 in.t \ lie compared to the elaeoblast of Pyrosoma and Salpa. As the 

 internal organs develop, five finger-shaped outgrowths appear on the 

 surface (Fig. 177, /) ;' these vary greatly in size and position and 

 degenerate later. They do not serve, as might be supposed, for the 

 fixation of the embryo. The nervous system (n) seems to develop 

 from an ectodermal depression. The primary enteric vesicle repre- 



