The eggs of Tunicata are for the most part of small size, nearly colourless and transparent, and with little or no food-yolk. {56}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 huxleyi_) 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, _t.c_). 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.[96] 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.