328 M. Ussow’s Zoologico-Embryological Investigations. 
Ill. The Body-wall. 
The envelope or wall of the body consists in all Tunicata 
of two contiguous mantles—an outer (tunica externa) and an 
inner one (tunica interna). 
In most of these animals (especially in the Chthonascidia) 
the outer mantle consists of three layers:—a, a peripheral 
layer (sometimes wanting) of spiniform cells (Cynthia) ; 6, a 
middle, more or less thick, gelatinous fundamental layer of 
rather firm coalesced sheaths (the so-called “ test-cells”’), pro- 
duced from the epithelial cells of the membrana granulosa of 
the Graafian follicle; and, c, a third layer, sometimes very 
thin and scarcely perceptible (e.g. in Ascidia intestinalis, 
canina; Salpe), which is composed exclusively of long elastic 
fibres, closely applied to the peripheral epithelial layer of the 
inner mantle (but never coalescent therewith). ‘The outer 
mantle of the Tunicata is not developed as a product of 
secretion of the epidermoidal cells of the inner mantle (Hert- 
wig*, ArsenjewT), but by the multiplication and growth of 
the above-mentioned “‘test-cells” (Kupfert, A. Kowalevsky$), 
which are at first arranged in a single layer between the yelk 
and the vitelline membrane (chorion). ‘The results of my in- 
vestigations of the formation of the so-called test-cells agree 
perfectly with those obtained by A. Kowalevsky||. The yellow 
corpuscles are in fact nothing but cells of the Graafian follicle, 
which have arranged themselves in a single series round the 
mature ovicell, and closed upon it before the formation of the 
chorion. In the larve of the simple Ascidia, and in the em- 
bryos of the Sa/pe and Pyrosomata, the rudiment of the outer 
mantle consists exclusively of radiating primitive yellow cells, 
rapidly multiplying by division, and their intercellular sub- 
stance. These cells, which put forth numerous processes 
and not unfrequently change their position (“ Wanderzellen,” 
Kolliker{]), soon coalesce with their sheaths. The contractile 
protoplasm of such cells gradually disappears (becomes ab- 
sorbed ?). The close network of coalesced thickened sheaths 
thus forms the porous, vesicular, fundamental substance of the 
inner mantle, always containing much water (especially in 
* Jen. Zeitschr. Band vii. 1872, p. 46. 
+ Berichte der Mosk. Ges. fiir Naturg. und Anthrop. Band x. 1872, 
p- 86; Jahresb. tiber die Anat. und Phys. 1873, Art. Tunicata, p. 307. 
t Schultze’s Archiv, Band vi. pp. 149, 159. 
§ Entwickelungsgesch. der einf. Ascid. pp. 13 e¢ seq. 
|| Schultze’s Archiv, Band vii. pp. 105 et seg. 
q Ann. Sci. Nat. tome v. p. 220; Mull. Arch. 1852, p. 325; Schultze’s 
Archiv, Band vi. p. 125. 
