REPORT 0]Sr THE TUKECATA. 
89 
strongly refracting germinal spot, botli circular in outline. The vitellus is coarsely 
granular. Externally the whole is enveloped in a cellular coating, the follicle, which 
is composed of rounded or nearly quadrangular nucleated cells. • When seen from the 
surface they are hexagonal or diamond-shaped in outline. A very delicately walled tube, 
which may be the oviduct, seems to run upwards to the atrium alongside the intestine, 
and to termiuate close to the anus ; on the other hand it may possibly be merely a fold of 
the mantle (see PI. YIl. fig. 7). 
Among the smaller ova there may be seen some still smaller triangular or pear- 
shaped masses of protoplasm with no nuclei, but coarsely granular in texture, which 
stain strongly vdth carmine (PI. VII. fig. 9). It is possible that these maybe small 
spermatic v’esicles, and such I believe them to be. If, however, they be merely young 
ova, then no trace of a testis has been discovered. Probably dichogamy exists here 
as in some other Compound Ascidians, and the colonies investigated were in their first 
sexual condition (protogynous), the testes being still in a rudimentary state. 
Incubatory Pouch . — This curious organ (PI. YIL figs. 7 and 12-14) is an appendage 
to the mantle, in connection with which the structure of its walls have already been 
described. It is merely an enormous diverticulum of the peribranchial or atrial cavity, in 
which the embryos lie during their development. It joins the atrium on its dorsal edge 
(PI. YIL fig. 7) close to the anus and the termination of the oviduct (?). It has a very 
narrow neck, barely wide enough to let a mature ovum pass in, and far too small to allow 
any of the embryos found in the pouch to go back again to the atrium. The problem 
then is, how do the fully developed larvae escape ? Two methods suggest themselves 
as possible : — Either (l)they burst their way through the outer wall of the colony, which 
is not very thick, when the larvae are fully developed and the incubatory pouch 
distended to its utmost, or (2) the entire pouch with its contained larvae remains intact 
till in the natural course of events, as will be seen shortly (p. 93), it reaches the 
summit of the colony and is cast off. The larvae may then be set at liberty by bursting 
the wall of the pouch or by its decay. 
Another curious point in regard to the incubatory pouch is that the embryos at its 
far end are always the youngest, while those near the neck are generally completely 
formed tailed larvae. Now if the ova, as they passed up into the peribranchial cavity, 
were deposited successively in the j)Ouch, those at the far end would be the most 
advanced in development, and mature ova or very young embryos only would be found 
at the neck of the pouch. Thus the observed arrangement is the reverse of wdiat 
would be naturally expected.^ If, how'ever, the ova are fertilized successively as they 
reach the peribranchial cavity, and then remain there for some time — until, in fact, they 
^ The greater part of this account of the stnrcture, &c., of Colella was written some years ago. Della Valle has since 
observed a similar condition of the incubatory pouch and its contained embryos and larvae in the genus Distaplia 
(Arch. ital. d. Biol., vol. i. p. 193). 
(ZOOL. CHALL. EXP. — PART XXXVIII. 1885.) 
Pp 12 
