638 



INVEETEBEATA 



CHAP. 



found in Botryllus and its allies ; and stolonial budding, found 

 amongst all other Tunicata. 



Stolonial budding has been thoroughly studied in the genus 

 Clavelina by Van Benedeu and Julin (1887), and the results of 

 these authors have been completely confirmed by Hjort's studies on 

 Perophora and Distaplia. In all three genera the two epicardial 

 cavities grow out from the pharynx, even in the free-swimming 



larva, and as soon as fixation takes place 

 they become long and fuse at their distal 

 ends into a single cavity. Van Beneden 

 and Julin imagined that the pericardium 

 was cut off from this cavity, but that, as 

 we have seen, is a mistake : the pericardium 

 originates earlier, as a median evagination 

 of the pharynx. The conjoined epicardial 

 cavities collapse, and their walls then 

 form a thin plate of cells, which projects 

 into a ventral protrusion of the ectoderm 

 which forms the stolon. It forms a 

 median septum in this stolon, which 

 divides the afferent and efferent blood- 

 streams from each other; but as this 

 septum does not reach quite to the tip, 

 the streams pass into each other here. 



New buds are formed as lateral 

 branches of the stolon. Branches of the 

 epicardial septum are continued into these 

 branches, and these pieces of the septum 

 develop into sacs by the reappearance 

 of the lost cavity of the epicardial tube 

 (Fig. 463). The young bud is therefore 

 formed from a double - walled vesicle, 

 of which the outer layer is furnished by 

 the parental ectoderm, whilst the inner 

 layer arises from the epicardial tube. 

 Between these two layers are contained 

 a certain number of loosely aggregated 

 mesoderm cells. 



As Hjort shows, the building up of 

 the organs of the young "blastozooid," 



as the "person" produced by budding is termed, takes place in 

 a very different manner from that in which it occurs in the 

 embryo produced from the fertilized egg. Thus the atrial cavity 

 arises by the downgrowth of two septa, which divide the inner 

 vesicle into a median and two lateral portions ; the median is the 

 rudiment of the pharynx, the lateral of the atrial cavities. The 

 nervous system arises as a dorsal hollow outgrowth of the inner vesicle, 

 at the border of the left and median divisions ; it grows forwards and 



FIG. 463. Budding stolon of 

 Perophora listeri showing the 

 development of buds from 

 the septum of the stolon. 

 (After Hjiirt.) 



6, blood ; e.l, external layer of bud 

 rudiment ; i.l, internal layer of bud 

 rudiment ; sang, blood-space ; isep, 

 septum of the stolon, a double layer 

 of cells. 



