No. 2.] TESr-CELLS OF ASCIDIANS. 1 97 



has passed into the general stroma of the ovary. This is the 

 stage in development just before the formation of the test- 

 cells. 



In the next figure (5) the test-cells have begun to appear. 

 At tc the follicular zone is seen pushing into the substance of 

 the ^%gi and a careful examination shows faint indications of 

 cell outlines at this place. In other words, one of the follicular 

 cells has changed somewhat its position, and has come to lie a 

 little interior to the cells of the follicular zone. The cell con- 

 tains a nucleus which agrees in all details with those of the 

 follicle. At tc^ another such cell is seen ; and at tc" a cell of 

 the follicular zone is seen pushing into the interior of the ^^2,. 



These three cells, which take a more internal position, are 

 the follicular cells, which are in process of conversion into test- 

 cells. Two main sources of error may arise in interpreting 

 the sections at this stage and are carefully to be avoided. 



Inner nuclei may be seen if the section passes — not through 

 the centre, but — near one end of the ^g^, where the convexity 

 of the surface is so great relatively to the plane of the section, 

 that two or more layers of the nuclei of the follicle may appear 

 in the same section. A careful count was made in all cases of 

 the sections and the central ones chosen for study. Again, an 

 error may arise when the microtome knife does not cut the egg 

 cleanly, but turns over part of the follicular zone. 



The stage in development in which the test-cells appear is 

 very constant, and I have never succeeded in finding any nuclei 

 or cells inside of the follicular zone before such a stage of 

 development is reached. In Fig. 6 we see a cross-section of 

 an egg of C. ocellata, in which the migration of test-cells is 

 completed. The follicular zone is seen to be divided up into 

 distinct cells, which are sharply separated from the Qg%. At 

 the periphery of the yolk, and touching the follicular zone, 

 lie the test-cells. These do not stain so deeply as the proto- 

 plasm of the Q.^^^ and in all respects resemble the cells of the 

 follicle. 



After a stage represented by Fig. 6 has been reached, the num- 

 ber of test-cells does not seem to increase ; although between 

 Figs. 5 and 6 it is probable that the test-cells divide at the 

 periphery of the yolk. 



In Fig. 6 there is seen at the outer periphery of the follicle 



