ASCIDIAE SIMPLICES AND COMPOSITAE. 



335 



surrounding water where they pass through their embryonic develop- 

 ment, being supported at the surface by the large, foam-like follicle- 

 cells (Fig. 149, '). Fertilisation usually takes place either in the 

 peribranchial (atrial) cavity or after the egg is laid, but exceptions 

 to this rule are found in the genera Cynthia and Lithonephrya 

 (GiAKD), these forms passing through their embryonic development 



Fig. 148. Three stages in the development of the egg of I'lmtli'^in maiiimillata (after. 

 KOWALEVSKY, adapted from KUPFPKR, FOL and others). , basal membrane of the 

 follicle ; f>, superficial layer of pavement-epithelium ; c, follicle-cells ; rf, chorion ; 



e. test-cells ; /'. egg-cell. 



within the peribranchial cavity of the mother. In Clavelina also, 

 and in all composite Ascidians, development up to the time when 

 the free-swimming, tailed larva is hatched takes place in the peri- 

 hranohial cavity of the mother, or else in peculiar diverticula of 

 this cavity known as brood-spaces. The composite Ascidians differ 

 from the solitary forms in the large amount of yolk contained in 

 the egg. SALENSKY (No. 49) recently observed in a few Polydimdae. 



