510 TUNICATA. 



In any case, the rudiment of the endostyle grows out as paired folds 

 projecting inward, while the intestinal loop (Fig. 268, d) is derived 

 from the lower end of the entoderm-vesicle, from which it grows out 

 as a blind diverticulum on the right side. 



The atrial cavity of the bud (Fig. 269, c) arises, according to 

 Brooks, through the union of the paired peribranchial tubes (Fig. 

 265, c). The cavity which thus arises then occupies a dorsal position 

 in the posterior part of the bud. Two lateral perforations which 

 occur in the partition-wall extending between the atrial cavity and 

 the pharynx represent the rudiment of the two gill-clefts, and the 

 trabecula remaining between the clefts is the rudiment of the gill 

 (Fig. 268, g), the ventral and lateral covering of which is derived 

 from the entoderm, while the covering of the dorsal side is yielded 

 by the epithelium of the atrial cavity [ectoderm, Brooks]. Only at 

 a later stage do the pharyngeal and atrial cavities open externally, 

 ectodermal invaginations leading to the formation of the branchial 

 and atrial apertures. 



The neural tube of the stolon becomes broken up into segments, 

 each of which, in the form of a spherical vesicle with thick walls, 

 gives rise to the central nervous system of a bud. In the young 

 buds, these appear remarkably large (Figs. 270, 275, n), but decrease 

 in size later. The vesicular rudiment which lies on the dorsal side 

 of the bud in the anterior region of the body, becomes divided into 

 two parts by a transverse furrow ; these parts at first remain con- 

 nected with each other, but are later completely separated. The 

 anterior part becomes connected with the entodermal wall of the 

 pharyngeal cavity and, after its lumen has broken through into that 

 cavity, it may be recognised as the rudiment of the ciliated pit. The 

 posterior part of the vesicle which soon loses its lumen is the rudi- 

 ment of the ganglion proper, in which a peripheral layer of ganglionic 

 cells and a central accumulation of punctate tissue (Leydig's Punkt- 

 substanz) develop. The peripheral nerve-strands also soon grow out. 

 A dorsal outgrowth from the ganglionic rudiment forms the rudiment 

 of the eyes which, in the buds, develop somewhat otherwise than in 

 the embryo. [See on the development of the eyes, Seeliobr (No. 105) 

 and Metcalf (Nos. 99, 99a, and I.).] 



The mesoderm of the stolon, which is represented by a mesenchyme 

 filling the primary body-cavity and by the Brooks' muscle-tubes, 

 gives rise to the connective tissue, the blood-vessels, the pericardial 

 vesicle, the elaeoblast and the body -uiusculature. We are not in a 

 position, however, to make any more definite statements as to the 



