REPORT OJS' THE TUHICATA. 
101 
represented in figure 6 is just ending its female stage of existence and preparing to 
function as a male. 
Incubatory Pouch. — In the present species this organ is so slightly developed that 
it is doubtful whether it should be considered as existing at all. The embryos merely 
lie in an enlargement of the base of the peribranchial cavity. 
On examining the thorax of a fully developed Ascidiozooid from the right side, two, 
three, or four large mature ova are noticed, forming a transverse row, which extends 
from the dorsal edge three-fourths of the way round the posterior end of the thorax 
towards the endostyle (PL XL fig. 2). Generally the dorsal end of the row projects 
more or less beyond the margin of the thorax, and is then contained in a sac-like diverti- 
culum of the mantle, havino’ no muscular bands. From the ventral end of this transverse 
row of ova a series of embryos in different stages of development extends in an oblique 
antero-posterior direction from the base of the endostyle towards the atrial aperture. 
The most posteriorly placed of these is only slightly more advanced in development than 
the ovum next it, while the embryo at the anterior end of the line is a tailed larva ready 
for expulsion (PL XL fig. 2). Thus a regular series is formed from the dorsal end of 
the sac across the posterior end and then along its length to the atrial aperture, the 
embryos being arranged in order of development. These series of ova and embryos 
merely lie in the peribranchial cavity, and are covered by the mantle, which they push 
out so as to form a sac-like diverticulum, which only differs from the incubatory pouch 
of the last species in degree of development and in not being so much constricted off 
from the rest of the peribranchial cavity. This arrangement rather supports the view 
(suggested on p. 89) that in Colella pedunculata the incubatory pouch may possibly be 
formed after the ova and embryos have all arrived in the peribranchial cavity. It is 
obvious that if in Colella thornsoni (see PL XL fig. 2) the region of the peribranchial 
cavity occupied by embryos were constricted off to form an incubatory pouch commencing 
with the posterior end of the dorsal edge, the youngest embryos would be placed at the 
far end of the pouch and the tailed larvae would be next the mouth — exactly the arrange- 
ment found in Colella p>edunciilata. 
The Process of Biiddmg. — In this species, as in the case of Colella peduncidata, gem- 
mation takes place in the peduncle. The vascular appendages have the same structure 
as in the last species. Each is divided by a median septum into two canals running 
side by side. The wall is formed of sc[uamous epithelium, the ectoderm, lined by a delicate 
layer of connective tissue. The ectodermal cells are polygonal and distinctly nucleated 
(PL XIII. fig. 5). These vessels and their lateral branches contain many blood-coiqDuscles 
— in some places the small vessels, and especially some short csecal processes which occur 
on their sides, are completely filled up with them (PL XIII. figs. 1, 4). Every here 
and there in the cavities of the vascular appendages occupying the larger canals of the 
stalk minute buds or spherical aggregations of cells are found (PL XIII. fig. 8 gm'). 
