338 TUNICATA. 



According to Chabry (No. 13), another fine, structureless membrane cover- 

 ing the externa] surface of the test-cells should be added to the above. In 

 the solitary Ascidians, after the egg is laid, the interspace between the chorion 

 and the surface of the egg is increased (Fig. 1-49) through imbibition of water 

 and consequent swelling of the gelatinous matter secreted at the surface of 

 the egg. 



We must here briefly allude to views as to the origin of the egg-envelopes 

 which differ from those given above. According to Sabatier, Fol (No. 21), 

 and Roule (No. 47) the follicle-cells are produced by the egg itself. Fol and 

 Roole think that this is brought about by a process of budding from the 

 germinal vesicle, but Sabatier considers that the follicle-cells arise through 

 free cell-formation in the yolk.* It seems to be fairly established that an 

 ejection of chromatin-elements actually takes place from the nucleus of the 

 ovum but we incline to the view that this process has nothing to do either 

 with the rise of the follicle-cells or with that of the test-cells. Such an 

 origin was assumed for the latter by Roule, Davidoff (No. 14), and Pizon 

 (No. XXVII.), while a number of other authors (Semper, Fol, Sabatier and 

 others, following Kupffer, No. 34) thought that the test-cells formed freely 

 in the protoplasm of the egg. 



It is of interest to note that the eggs of Appendicularia also are enveloped 

 while in the ovary by a follicle (A. B. Lee, Davidoff). When laid, however, 

 they are without covering, and only after fertilisation has taken place in the 

 water do they become surrounded by a delicate vitelline membrane (Fol, 

 No. 21). 



B. Cleavage. 



The free-swimming, tailed Ascidian larvae were known to the older 

 authors and were carefully described by Milne-Edwards, P. J. 

 van Beneden, and others. Our first knowledge of the embryonic 

 development of the Ascidians, however, was due to the researches of 

 Kowalevsky (Nos. 29 and 30), which were soon supplemented by 

 the accounts of Kupffer (Nos. 34 and 35) and Metschnikoff (No. 

 41). Among later workers we must mention Seeliger (No. 50) and 

 van Beneden and Julin (Nos. 7-10) ; the eggs of the Ascidiae 

 compositae which are rich in yolk have been investigated by Maurice 

 and Schulgin (No. 39) and Davidoff (No. 14).f 



Th'e cleavage of the Ascidian egg is total, and, seeing that the 

 blastomeres at first differ only to an inconsiderable extent in size and 

 structure, may be described as almost equal. The term " adequal 



* [As we have already stated, free cell-formation is now generally discredited 

 by cytologists. — Ed!] 



t [Still more recently, Salensky (No. XXIX.) has worked at the develop- 

 ment of the Ascidiae compositae. Castle (No. II.) has worked at Ciona 

 intestinaUs. The work of the last-mentioned observer is very important and 

 exhaustive. — Ed.] 



