56 ASCIDIANS 



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 mentulci) 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, i.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. 1 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 (Jiy^. 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 



1 The early stages of dona, of which Castle has given a very complete account 

 (Bull. Mus. Comp. Zool. xxvii. No. 7, 1896), differ iu some points from those of 

 Ascidia described here. 



