336 TUNICATA. 



(Amarotieittm, Circinalium, Fragarium.) a fusion of the embryo with 

 the atrial wall of the mother. At this spot a thickening, the placenta, 

 is formed, derived in part from the atrial wall (placenta materna), 

 from the follicle-epithelium enveloping the embryo and from an 

 ■accumulation of test-cells (kalymmocytes). 



These Ascidiacea are hermaphrodite. Self -fertilisation seems, in- 

 deed, to be prevented, in most cases, by the maturation at different 

 times of the male and the female genital products, but is not im- 

 possible in other cases, in which both products ripen simultaneously. 

 [In Ciona (Castle, No. II.), although the products ripen at the same 

 time, self-fertilisation does not occur.] 



The mature eggs are surrounded, at the commencement of em- 

 bryonic development, by a complicated system of envelopes, which 

 we are inclined to regard as derivatives of the original egg-follicle. 

 In this respect we agree with Kowalevsky, whose views were con- 

 firmed later by Van Beneden and Julin (No. 10) as well as by 

 Morgan (No. 46), but we must point out that the origin of these 

 envelopes is still an open question. This point will be further dis- 

 cussed in the section on the formation of the egg under the heading 

 of general considerations. 



Very young eggs, while still in the ovary, appear to be surrounded 

 by a pavement-epithelium consisting of flattened cells (Fig. 148 A, c). 

 The elements of this primary follicular epithelium (c) are derived 

 from undifferentiated cells of the ovary (Van Beneden and Julin). 

 A structureless basal membrane (a) seems to be secreted early at the 

 surface of the follicular epithelium. The follicle-cells multiply by 

 division, and soon lose their flattened form and become cubical ; a 

 few of these cells ai-e displaced inwai'ds (Fig. 148 B, e) in such a 

 way as to become deposited on the surface of the egg into which 

 they may even find their way, passing into the most superficial layer 

 of the vitellus. These cells, which can usually be distinguished by 

 their yellow colour, have been called test-cells (<■) because it was 

 erroneously supposed that they gave rise to the cells of the cellulose 

 mantle (test) of the adult Ascidian, a view which was refuted by 

 O. Hertwig (No. 25).* These test-cells, which soon increase very 



* [These are the abortive eggs (Abortiveier) of Davidopp (No. 14) and the 

 kalynvmocytes of more recent authors. There seems to be much uncertainty 

 about the fate of these cells. Salensky (No. XXIX.), from his investigations 

 on the compound Aseidiaus, regards them as giving rise to the test (see p. 

 356) whereas the similarly named cells of Sal/pa appear to be transitory 

 nutritive structures (Heider, No. XIII. ; Korotneff, No. XVIII. ; Metcalf, 

 No. XXIV. ; Pizon, No. XXVII.), the most recent investigator of the origin of 

 the follicle- and test-cells seems to agree with the account given above. — Ed.] 



