S9 



coarser tubules of the ovary and the adjacent parts of the 

 intestinal wall. These spermatic tubules terminate in 

 ovate swellings, usually grouped in bunches (PI. IV., 

 fig. 15). Near the commencement of the rectum the larger 

 tubules unite to form the vas deferens, a tube of consider- 

 able size which runs forward alongside the rectum, and, 

 like the oviduct, terminates by opening into the peri- 

 branchial cavity close to the anus. The lumen of the 

 tubules of the testis, like the cavity of the ovary, is a meso- 

 blastic space in the embryo, and the spermatozoa are 

 formed from the cells lining the wall, and are set free into 

 the cavity. 



The mature ovum is of small size (about 0"12 mm. in 

 diaui.), colourless, and with little or no food yolk in the case 

 oi Ascidia. It is only some of the germinal cells in the 

 ovary that are destined to become ova. Of the rest, some 

 form a protecting layer, the follicle, around the young ova. 

 Certain of these primary follicle cells migrate inwards and 

 give rise, by proliferation, to a layer of cells in the superficial 

 part of the ovum (PI. V., fig. 1, ^.c). These are the 

 so-called "testa-cells" or kalymmocytes. These later on 

 produce a thin gelatinous layer over the surface of the 

 ovum and between it and the follicle, which looks like 

 the beginning of the test — hence the name given to the 

 cells, which, although so different in origin from the 

 mesodermal test cells of the adult, probably to some 

 extent give rise to " bladder cells " in the test. The rest 

 of the follicular " testa-cells " eventually disappear. 



The follicle cells proper produce two layers, the outer of 

 which remains in the wall of the ovary when the ovum is 

 set free ; while the inner layer adheres to the surface (PI. 

 v., fig. l,foU.), and its cells become large and much vacuo- 

 lated, some of them growing out to form long papillae, 

 which help to sustain the floating egg in the sea-water. 



