xiii PHYLUM CHORDATA 27 



close relation to the alimentary canal in the " nucleus," and their 

 short ducts open into the peribranchial cavity. In Doliolum the 

 elongated testis and oval ovary have a similar position to that 

 which they occupy in Salpa, but the ovary consists of a number 

 of ova. 



Development and Metamorphosis. Usually impregnation 

 takes place after the ova have passed out from the atrial cavity. 

 But in a few simple and many compound forms impregnation 

 takes place in the atrium, and the ovum remains there until the 

 tailed larval stage is attained. In. certain composite forms there 

 is a coalescence of the embryo with the wall of the atrium, forming 

 a structure analogous to the placenta of the Mammals and desig- 

 nated by that term. Self-impregnation is usually rendered im- 

 possible by ova and sperms becoming mature at different times ; 

 but sometimes both become ripe simultaneously, and self-im- 

 pregnation is then possible. 



A somewhat complicated series of membranes invests the ovum. 

 The immature ovarian ovum is enclosed in a layer of flat cells the 

 primitive follicle cells derived from indifferent cells of the ovary. 

 On the surface 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 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 testa cells : they afterwards develop on the outer surface a 

 thin structureless layer, the chorion. Meantime, 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 tho follicle cells becomes 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 ventral side, are the beginnings of the 

 ectoderm ; the four larger form the endoderm, but also perhaps give 

 origin to a number of small ectoderm cells. A small segmentation- 

 cavity (Fig. 688, A, scg. cav.*) appears early. A curvature of the 

 embryo then supervenes, so that the side on which the larger 

 cells are situated becomes concave, and the larger cells thus 

 become invaginated within the smaller, obliterating the segmenta- 

 tion cavity, the result being the formation of a gastrula stage (B) 

 with an archenteron. The blastopore, at first very wide, gradually 

 becomes narrowed to a comparatively small rounded aperture (6') 



