410 TUNICATA. 



foration which takes place between the neural and the enteric 

 cavities, becomes the rudiment of the so-called ciliated pit (Fig. 200, 

 ff). According to Salensky, the lumen of the neural tube is obliter- 

 ated later, through the encroachment of the cells forming its walls, 

 as the ganglion develops further. The lumen of the embryonic ciliated 

 pit is also said to disappear ; the similarly-named organ of the adult 

 is thought to arise anew from the enteric wall. The blind end of the 

 ciliated pit changes later into a large sac lying beneath the ganglion 

 which probably forms the rudiment of the sub-neural gland. 



Since the chain of Ascidiozooids arose as an outgrowth from the 

 body of the Cyathozooid, its primary body-cavity, which extends from 

 the enteric tube and the peribronchial cavities to the ectoderm, is in 

 open communication with the primary body-cavity of the Cyathozooid. 

 It is therefore possible for mesenchyme-elements to pass over from 

 the Cyathozooid into the chain of Ascidiozooids. According to 

 Salensky, this actually happens to a great extent, large numbers of 

 the elements of the cell-zone passing over into the chain. According 

 to this author, the cell-zone which was described above (p. 398), and 

 which is to some extent derived from inner follicle-cells (kalymmocytes), 

 but for the greater part from elements of the disintegrated left coelemic 

 sac, is the principal source of the whole of the mesoderm in the Asci- 

 diozooids. It must indeed be pointed out that the pericardial tube 

 which is derived from the right coelomic sac and extends posteriorly 

 as a prolongation of the pericardial rudiment of the Cyathozooid, after 

 breaking up into separate cells, may also contribute to the formation 

 of the mesenchyme in the Ascidiozooids, and this may also be the 

 case with the so-called axial mesoderm-strand which arose from the 

 tube interpreted as the remains of the chorda. The later fate of 

 these structures is difficult to make out from Salensky's account. 

 This author in any case seeks the origin of the mesoderm of the 

 Ascidiozooids in the cell-zone which, in its turn, is derived principally 

 from the elements of the disintegrated left coelomic sac. The immi- 

 gration of the mesoderm into the germ-stock (the chain of Ascidio- 

 zooids) takes place first in the form of an ingrowth of crowded 

 cell-masses. Later, when the cell-zone has broken up into separate 

 islands, detached cell-groups or single elements pass over into the 

 body-cavity of the Ascidiozooids. 



The mesodermal elements become distributed in the primary body- 

 cavity of the Ascidiozooids. Two groups of them, however, soon take 

 up a definite position in the posterior region of the zooid at either 

 side of the body, and their elements are found to be arranged in two 



