102 
THE VOYAGE OF H.M.S. CHALLENGER. 
These are from 0'02 to 0‘06 mm. in diameter, hut no larger (see PL XIII. fig. 1). 
They are of a yellowish-brown colour, and so opaque that no structure can be made out 
in their interior. The method of formation of these young buds was not directly observed, 
but in all probability they arise as in the case of the last species by the aggregation and 
proliferation of a number of the blood corpuscles surrounded by an ectodermal layer 
derived from the wall of the vascular appendage. The only specimen showing a stage in 
the early formation of a bud is the one represented in figure 9, where an outer layer of 
cells is seen separating off from the ectodermal wall of the vessel (ec.), while the free 
mesoderm cells inside are becoming arranged along the wall and in some places are 
proliferating. Some of these cells have become enlarged (o.) and already look like the 
ova which they become later on. Just as in Sarcobotrylloides ivyvillii, young ova are 
conspicuous in the bud from a very early period (PI. XIII. figs. 7, 8). 
In transverse sections of the stalk, until the very top is reached, no further develop- 
ment of the buds is seen except a slight increase in size. Just below the head, however, 
there is a narrow zone where buds are found in various stages of development from the 
simple spherical double-walled sac up to the young Ascidiozooid (PI. XIII. figs. 7, 8). 
The course of development is apparently the same as in the last species. Sections 
showing the archenteron divided into three, the dorsal nerve tube, the ventral endostyle, 
the posterior ovary, and lastly the elongation of the abdominal region may all be seen. 
It is remarkable, however, that the earlier stages, showing the division of the archenteron 
and the formation of the endostyle, which were those most commonly seen in sections of 
the peduncle of the last species, are here much rarer than the later stages, where the 
ovary and intestine have made their appearance (see PI. XIII. fig. 7). 
The buds in this species seem to remain, when in the condition of two membranes 
surrounding an archenteron, in a dormant condition undergoing no change except a 
slight increase in size, until they have reached the top of the stalk, where they rapidly 
pass through the stages of their development which had been delayed, and appear in the 
form of young Ascidiozooids (PI. XIII. fig. 10) which then continue to grow slowly in 
size until they arrive at the base of the head. This course of development would explain 
the comparative rarity of buds in the earlier stages, which were passed through more 
rapidly. In some cases the buds do not become completely constricted off from the 
vessels as in Colella 'pedunculata, but remain attached by their posterior ends up to a late 
period in their development (see PI. XIII. fig. 8). Figure 10 shows a young Ascidio- 
zooid in which all the chief systems of the body, including the genital glands (ov. and t.v.), 
and even the digestive gland {h. gl.) on the intestine, are already preseut. 
The Formation of the Colony. — In this species the colony seems to be more stable and 
to undergo change less ra^^idly than in the case of Colella pedunculata. This may be 
inferred from the observed facts, that the Ascidiozooids are more of the same size, the 
differences between those at the opposite ends of the head not being very great ; that 
