ASCIDIACEA DEVELOPMENT OF THE FREE-SWIMMING LARVA. 357 



mantle play an 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. Phallu.na, 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. HEBTWIG 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 Distaplia 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, 

 nr, 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 or sensory vesicle (Fig. 167, sb, vesicule 

 'infrrifiure ou cerebrals of VAN BENEDEN and JULIN) while the 

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

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

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

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

 'In //<////' 'ncephale 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. 



Thi' rr/r/y/v// or senxory 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, 

 No. 7). The two organs known as the eye and the otocyst (au and ot, 

 Fig. 1 68) scon appear in the form of accumulations of pigment. The 

 eye, which is derived from the right dorsal swelling (Fig. 168 B), 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). 



