ASCIDIAE SIMPLICES AND COMPOSITAE. 



337 



derived from the 

 be described 



FIG. 149. Mature egg from the oviduct 

 of Ascidia canina * (after KUPPFEB). c, 

 follicle-cells (foam-like cells) ; d, chorion ; 

 e, test-cells ; /, egg-cell ; x, gelatinous 

 substance. 



greatly in number, at first form an inner epithelial layer over the 



surface of the egg known as the twf-wll-lnyer (Fig. 148 (7, e). In 



later stages they undergo a process of degeneration. They then lose 



the regularity of their arrange- 



ment and are found embedded 



separately in a gelatinous mass 



secreted over the surface of the 



egg. Their original cellular 



character is then less distinct 



and has been altogether denied 



by some authors (SEMPER, FOL). 



After the development of the 



test-cell-layer, a structureless 



membrane (Figs. 148, 149, d) 



is secreted between it and the 



actual follicular epithelium, and 



this, being 



follicle-cells, may 



as the I'/ioi'ion. 



In the meantime, a delicate e.rternal pavement-epithelium (Fig. 

 148 C, I) has developed on the outer surface of the follicle which 

 is in close contact with the basal membrane : this epithelium is pro- 

 bably to be regarded as an outer layer of follicle-cells. Like the 

 basal membrane, it seems to disappear, not being found in eggs after 

 they have been laid. 



In the free eggs of the solitary Ascidians, the cells of the actual 

 follicle-layer, at a later stage, assume a peculiar character. Their 

 protoplasm becomes filled with numerous vacuoles (Fig. 148 C, 149, c) 

 and thus resembles foam. The cells, which increase in size and form 

 papilla-like structures (Fig. 149, c), are concerned in supporting the 

 \irjr as it floats in the water. 



The mature Ascidian egg thus possesses the following envelopes, 

 reckoned from without inwards : 



(a) The basal membrane of the follicle. 



(b) The external pavement-epithelium. 



(c) The actual follicle-epithelium (layer of foam-like cells). 

 ('/) The chorion. 



(e) The layer of test-cells [kalymmocytes, abortive eggs] . 



Ciona intestinalis, var. ED.] 



