No. 2.] TEST-CELLS OF ASCWIANS. I99 



from sections, it seems to me to leave not the slightest doubt as 

 to the origin of the test-cells. The protoplasm of the egg of 

 Cynthia stains deeply, and the color cannot easily be extracted. 

 In this respect it is unfavorable material to determine the truth 

 of Davidoff's position ; but in the next form, Clavellina, the pro- 

 toplasm (and yolk) stains scarcely at all, and v^andering nuclei 

 could be easily observed were there any such in the egg between 

 the nucleus and the follicle. 



Clavellina sp.f — The ascidian from which the material for 

 work was obtained was collected at Green Turtle Cay, Ba- 

 hamas, but I have not identified the species. The eggs are 

 extremely large and very favorable for study, both on ac- 

 count of their size and for other peculiarities which will be 

 seen in the following description. On account of the large 

 amount of food yolk in this American Clavellina, no better 

 material could be obtained to test the correctness of Davidoff's 

 theory. The test-cells are not formed till the egg is quite large, 

 and has a good deal of yolk ; so that if the nucleus of the test- 

 cells came from the nucleus of the ovum, these migrating pieces 

 would have to travel a long distance from the centre to the 

 periphery of the egg, and the chances of seeing such a migra- 

 tion would be very great. 



The young nuclei of the ovum arise in the germinal epi- 

 thelium of the ovary, and are at first like the ordinary nuclei of 

 the membrane. The earliest stage is shown in Fig. 11. A 

 somewhat older ovum is shown in Fig. 12, in which the proto- 

 plasm about the nucleus has increased, and there is a sharper 

 distinction between the egg proper and the more peripherally 

 lying follicular nuclei. The protoplasm of the follicular nuclei 

 is seen to be thickening over the surface of the ovum, beginning 

 at that part of the periphery of the egg which is in contact with 

 the germinal membrane. In Fig. 13 the ovum has left the 

 surface of the cavity of the ovary and passed into its substance. 

 The peripheral zone of follicular protoplasm has spread over the 

 whole surface of the ovum, and the follicular nuclei, which are 

 now more numerous than in the last figure, lie within this proto- 

 plasm. At this stage in the history of the ovum of Clavellina 

 the yolk commences to form, and appears as a zone around the 

 nucleus. It is seen in Fig. 12 as an irregular mass, which in 

 the figure is colored a lighter shade than the surrounding 



