150 ^ESTIVATION OF BOTRYLLOIDES GASCOI DELLA VALLE. 



continued growing, and had been occupying new parts of the slide. Two portions 

 could now be distinguished: first, a thick opaque part in which the ampullae were 

 visible only at the edges and on the surface by reflected light, and on which no buds 

 could be seen; and secondly, a thin translucent part, with larger and fewer ampullae 

 and some good-sized buds. The smaller of these buds were the size of ampullae and 

 could hardly be distinguished from them. They looked as if they were being devel- 

 oped from ampullae, and half of the colony was preserved and sectioned to see if this 

 actually was the case. 



II. MINUTE ANATOMY. 



The sections showed many small buds scattered all through the colony, even in 

 the opaque part, but no transitions between ampullae or masses of blood-cells and 

 buds. The smallest buds had inner vesicles less than 30 micra in diameter (PL XI, 

 Figs. 10, 11), while the largest ones were zooids which were nearly ready to open their 

 siphons and had begun to produce buds. In all cases the cell-layer that corresponded 

 to the outer vesicle or ectoderm of the bud was a continuation of the wall of some 

 blood-vessel or ampulla. But the relation of this outer cell-layer to the inner vesicle 

 was very variable. Frequently this layer was closely applied to the inner vesicle 

 (Fig. 11). Often there was between the two a wide space filled with blood-cells, thus 

 giving the impression that the inner vesicle was contained in an ampulla (Fig. 

 12). Finally, in two cases, vesicles were seen that were not at the end of one of the 

 branches of the vascular system, but rested against the wall of a vessel as if fixed in 

 transit (Fig. 10). From these observations it seems probable that some of the inner 

 vesicles of the young buds are transported from place to place by the blood current 

 and develop wherever they happen to lodge. Thus, properly speaking, the buds 

 cannot be said to have any outer vesicle at all, especially in their early stages, but 

 wherever they begin to* develop, the ectodermic wall of the vascular system develops 

 into the ectoderm of the bud. 



This conchi sion is of some general importance in view of the explanations that 

 have been offered for the differences between the oogenic and blastogenic ascidian 

 development. Thus Seeliger ('84), Hjort ('95), and Bitter ('96, p. 210) have said 

 that the reason why, in the bud, the ectoderm takes no part in the development 

 is because it is not an embryonic but a differentiated cell-layer with a special function 

 to perform. It is already a functional part of the ectoderm of the adult before it 

 becomes the outer vesicle of the bud. To this argument it might possibly be objected 

 that there is no reason why a portion of the ectoderm (for instance, that over the bud- 



