?.9. 



;^0(>LOGY 



Fin. VS^.— Aficidian (Ciona). Mature egg 

 from the oviduct, c. follicle-cells ; .', 

 chorion ; e, test-cells *, /, ovura ; x, gela- 

 tinous layer. (From Korschelt and Heider, 

 after Kupffer.) 



becomiiig mature at different times; but sometimes both become 

 rijie simnltaneousl)', and self-impregnation is then possible. 



A somewhat complicated scries of membranes (Fig. 732) invests 

 the ovum. The immature ovarian ovum is enclosed in a layer of 



flat cells — the primitive fullide- 

 cclls — derived from indifferent 

 cells of the ovary. On the sur- 

 face of this is developed a 

 structureless basal membrane. 

 The follicle cells increase by 

 division and soon form a sphere 

 of cubical cells. Certain of the 

 cells migrate into the interior 

 of the sphere so as to form a 

 layer on the surface of the ovum. 

 Others penetrate int(j the latter 

 so as to lie in the superficial 

 strata of the yolk. The layer 

 of cells on the surface of the 

 ovum are termed the test-cells 

 {() : they afterwards develop 

 on the outer surface a thin 

 structureless layer, the chorion, and internal to them is formed 

 a gelatinous layer (.^') through which the test-cells in a de- 

 generated condition become scattered. Meantime, external to 

 the follicle-cells, between them and the basal membrane, has 

 ajipeared a layer of flattened epithelial cells ; this, with the basal 

 membrane, is lost before the egg is discharged. In all the simple 

 Ascidians, with the exception of the few in which development takes 

 place internally, the protoplasm of the follicle-cells (Fig. 732, c) 

 is greatl}' vacuolated so as to appear frothy, and the cells 

 become greatly enlarged, projecting like papillae on the surface and 

 buoying up the developing ovum. 



Segmentation is complete and approximately equal, but in the 

 eight-cell stage four of the cells are smaller and four larger : 

 the smaller, situated on the future dorsal side, are the beginning 

 of the endoderm ; the four larger form the greater part, if not the 

 whole, of the ectoderm. In the following stages the ectoderm cells 

 multiply more rapidly than the endoderm, so that they soon 

 become the smaller. In the sixteen-celled stage the embryo (Fig. 

 733, A) has the form of a flattened blastula (plaeula) with ectoderm 

 on one .side and endoderm on the other, and with a small segmenta- 

 tation- cavity. The transition to the gastrula stage is in most 

 Ascidians effected by a process intermediate in character 

 between embolic and epibolic invagination ; in some the invagina- 

 tion IS of a distinctly epibolic character. In the former case the 

 ectoderm cells continue to increase more rapidly than the 



