56 ASCIDIANS CHAP.. 
In some, however (such as some of the Cynthiidae, and some 
Compound Ascidians), the eggs are larger, more opaque, and have 
a fair amount of food-yolk. Ova of this type are not expelled 
from the body of the parent as ova, but are fertilised, and remain 
in the atrial cavity or in a special diverticulum thereof—the 
incubatory pouch—until they are far advanced in development ; 
and usually leave the body as tailed larvae. In many species, the 
ova and spermatozoa mature at different times in the life-history, 
and so self-fertilisation is prevented. Some species (such as 
many Botryllidae and Distomatidae) are protogynous, the ova 
being produced and shed before the testes have matured, while 
other species (Coelocormus hualeyi) are protandrous, being male 
while young and female later. But there is no doubt that in 
other cases (e.g. Ascidia mentula) self-fertilisation is not only 
possible, but does take place. After maturation certain of the 
follicle-cells which invest the ovum in the ovary migrate into the 
egg and proliferate so as to form a layer in the superficial part 
of the egg, where they appear as the so-called “ testa-cells” or 
“kalymmocytes” (Fig. 25, A, 4c). The remaining follicle-cells 
may form two or more layers, usually one of large cubical cells, 
which may become greatly vacuolated, next to the ovum, and an 
external flattened layer which is cast off when the egg escapes 
from the ovary. 
Segmentation is complete and results in the formation of a 
spherical blastula with a small segmentation-cavity (Fig. 25, C). 
The blastula grows larger and begins to differentiate! There 
are slightly smaller cells which divide more rapidly at one end of 
this embryo, the future ectoderm, and slightly larger and more 
granular cells at the other, which become chiefly endoderm 
(hypoblast). Invagination of the larger cells then takes place 
(Fig. 25, D), resulting in the formation of a gastrula with an 
archenteron. The hypoblast cells lining the archenteron become 
columnar (hy). The curving and more rapid growth at the 
anterior end of the embryo narrow the primitively wide open 
blastopore, and carry it to the posterior end of the future dorsal 
surface (Fig. 25, E). The orientation of the body is now clear. 
The embryo is elongated antero-posteriorly, the dorsal surface is 
The early stages of Ciona, of which Castle has given a very complete account 
(Bull. Mus. Comp. Zool. xxvii. No. 7, 1896), differ in some points from those of 
Ascidia described here. 
