xin 



PHYLUM CHORDATA 



31 



interior of the sphere so as to form a layer on the surface of the 

 ovum. Others penetrate into 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 (Fig. 741, e) : they afterwards develop on 

 the outer surface a thin structureless layer, the chorion (d), and 

 internal to them is formed a gelatinous layer (x) through which 

 the test-cells in a degenerated condition become scattered. Mean- 

 time, external to the follicle-cells, between them and the basal 

 membrane, has appeared 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. 741, c) is greatly 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 be- 

 ginning of the endoderm ; the 

 four larger form the greater 

 part, if not the whole, of the 

 ectoderm. > In the following 

 stages the ectoderm cells multi- 

 ply more rapidly than the 

 endoderm, so that they soon 

 become the smaller. In the 

 sixteen-celled stage the em- 

 bryo (Fig. 742, A) has the 

 form of a flattened blastula 

 (placula) with ectoderm on 



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

 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 invagination is 

 of a distinctly epibolic character. In the former case the ectoderm 

 cells continue to increase more rapidly than the endoderm, the 

 whole embryo becomes curved, with the concavity on the endodermal 

 side, and the ectoderm extends over the endoderm, the two layers 

 coming to lie in close contact and the segmentation-cavity thus 

 becoming obliterated. The concavity deepens until the embryo 

 assumes the form of a saucer-shaped gastrula with an archenteron 

 and a blastopore which is at first a very wide aperture extending 

 along the whole of the future dorsal side. The blastopore gradually 

 becomes constricted (Fig. 742, B) the closure taking place from 



FIG. 741. Ascidian (Ciona). Mature egg 

 from the oviduct after the basal membrane 

 and layer of flattened cells have been 

 thrown off. c. folJicle-cells ; d, chorion ; 

 e, test-cells ; /, ovum ; a-, gelatinous layer. 

 (From Korschelt and Heider, after Kupffer.) 



