398 TUNICATA. 



The germ-disc appears surrounded by a semicircular zone of cells 

 lying on the surface of the yolk (Fig. 192, z) ; KOWALEVSKY believes 

 that this zone consists of immigrated cells, the so-called inner follicle- 

 cells or kalymmocytes, but SALENSKY holds that mesodermal elements 

 (corresponding to the disintegrated, left coelomic sac) also contribute 

 largely to it. When, at a later stage, the surface of the yolk is 

 grown over by the continually extending germ-disc, this cell-zone 

 also passes under the ectoderm of the disc and thus into the primary 

 body-cavity of the Cyathozooid (Fig. 193 B, z). It then soon breaks 

 up into separate islands (Fig. 194, z) which are still for a long time 

 visible near the surface of the food-yolk. KOWALEVSKY thought that 

 the elements of this cell-zone took no further part in the formation 

 of the embryo, or at the most changed into blood -corpuscles, but 

 SALENSKY ascribes to them a very important part in the development 

 of the mesoderm of the Ascidiozooid (see below, p 410). 



C. The Development of the Primary Tetrazooid Colony. 



The edge of the germ-disc, by continually extending, overgrows 

 the yolk-sphere (Figs. 193 B, 194), which was originally covered 

 merely by the follicular epithelium. The food-yolk in this way 

 comes to lie inside the Cyathozooid, i.e., in its body-cavity. No 

 part is taken in this circumcrescence, however, by the posterior region 

 of the elongated disc (Fig. 192 B, x). This soon protrudes and 

 grows out into a long sac-like appendage (Fig. 193) which is cut iip 

 by transverse furrows into four sections (recalling the strobilation of 

 the tape-worm) ; these sections are the rudiments of the first four 

 Ascidiozooids. This chain of Ascidiozooids, which is known as the 

 stolon, and is evidently homologous with the ventral stolon of Doliolwni 

 and Salpa, is originally straight, lying parallel to the principal axis 

 of the Cyathozooid (Fig. 193 A). Later, however, as it lengthens, 

 it curves and finally lies equatorially (Figs. 194, 195, 196) so that 

 the Ascidiozooids form a ring surrounding the gradually diminishing 

 Cyathozooid. The individual Ascidiozooids at the same time alter 

 their positions ; at first they lay with their longitudinal axes in the 

 same direction as that of the whole stolon (Fig. 194), but later there 

 is a tendency for these axes to lie parallel to the principal axis of the 

 Cyathozooid (Fig. 196). The stolon then, as a whole, forms a series 

 of zig-zags, as the thin, drawn out trabeculae (Fig. 196, , s) con- 

 necting the individual Ascidiozooids lie obliquely, ascending from the 

 posterior end of one zooid to the anterior end of the next, 



