ASCIDIACEA — DEVELOPMENT OF THE FREE-SWIMMING LARVA. 367 



mantle play art important part in the process (Maurice, No. 40). 

 The histological character of the mantle-tissue may undergo further 

 modification, such as the vesicular transformation of the mantle-cells 

 ii. Phalltma, and the appearance of fibrillae in the ground-substance 

 in Cynthia. 



Since the surface of the embryo is, from the earliest stage, surrounded by 

 a gelatinous covering, in which lie embedded the yellow test-cells, it was 

 formerly thought that this layer was to be regarded as the rudiment of the 

 future mantle (Kowalevsky, Kupffer), an error to which the test-cells owe 

 their name. Zoologists were inclined to consider the mantle of the Ascidians 

 as a persisting embryonic envelope. O. Hertwig first proved that the test- 

 cell-layer is lost and that the mantle arises from the ectoderm. The im- 

 migration of the mantle-cells was only recently observed by Kowalevsky. 

 Salensky, however, in a recent treatise (No. 49, also No. XXIX.) has returned 

 to the older view, ascribing to the test-cells (kalymmocytes) in Bistaplia the 

 principal part in the formation of the cellulose mantle [see footnote, p. 336.] 



The nervous system. The rudiment of the central nervous system 

 which has hitherto been called the medullary tube, from the early 

 stages onward, shows a dilatation of its anterior section (Fig. 163, 

 >n\ p. 351). In the later stages which lead to the development of 

 the free-swimming larva, this dilated anterior part gives rise to a 

 vesicle, the so-called cerebral <>r sensory vesicle (Fig. 167, sb, vesicuh. 

 ante'rieure ou cere'brale of van Beneden and Julin) while the 

 posterior, narrowed part yields the caudal section (region caudate) of 

 the nerve-cord (s). These two parts appear connected by a middle 

 part (/■) with a narrow central canal and thickened wall which 

 Kowalevsky (No. 30) has called the trunk-ganglion (portion viscerale 

 'in myelencephale of van Beneden and Julin). The former con- 

 nection between the neural tube and the exterior (the neuropore) 

 completely closes even before the appearance of the oral aperture, 

 which lies near the same point. 



The <■> ,■< i>rnl or sensory vesicle represents the most anterior part of 

 the medullary tube swollen out into a vesicle by the dilatation of its 

 central canal. Its walls consist for the most part of pavement- 

 epithelium, but the dorsal wall is thickened and divided into a right 

 and a left swelling by a median furrow (van Beneden and Julin, 

 Xo. 7). The two organs known as the eye and the otocyst {on and -</, 

 Fig. 168) soon appear in the form of accumulations of pigment. The 

 eye, which is derived from the right dorsal swelling (Fig. 168 />'), is 

 a cup-like deposit of pigment at the inner ends of several radially 

 placed columnar cells, the cavity being occupied by a lens with a 

 superimposed meniscus (Fig. 168). 



