ASCIDIAE SIMPLICES AND COMPOSITAE. 



337 



Fig. 149. — Mature egg from the oviduct 

 of Asddia canina * (after Kutffer). c, 

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

 e, test-cells ; f, egg-cell ; ./■, gelatinous 

 substance. 



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

 surface of the egir known as the test-cell-layer (Fig. 148 C, 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 nuiss 

 secreted over the surface of the 

 egg. Their original cellular 

 character is then less distinct 

 and has been altogether denied 

 by some authors (Sempek, Fol). 

 After the development of the 

 test-cell-layer, a structureless 

 membrane (Figs. 148, 149, <l) 

 is secreted between it and the 

 actual follicular epithelium, and 

 this, being derived from the 

 follicle-cells, may be described 

 as the chorion. 



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

 148 C, h) 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 tilled 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 

 egg 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. 



(^) The layer of test-cells [kalymmocytes, abortive eggsj . 



#[ = Ciona intestmalis, var. — Ed.] 

 z 



