No. 2.] TEST-CELLS OF ASCIDIANS. 203 



ular cells by a process in which some of the cells are pushed 

 interior to the others is easily understood. We see in the ovo- 

 genesis of the squid a parallel case, where the process is carried 

 to even a greater extent, the substance of the ovum being per- 

 meated by the trabecular of the follicular zone. In the squid 

 there being many cells pushed in at several points, and in the 

 Ascidian a single cell here and there over the surface. 



The origin of the ova in vertebrates at the surface of the 

 ovary, and their migration into its stroma, accompanied by 

 follicular cells, themselves derived from the peripheral cells of 

 the ovary, and their subsequent arrangement around the young 

 Q^g into two or more layers, furnishes a most striking resem- 

 blance to the origin of ova, and follicular and test-cells, of the 

 Ascidians. 



In Davidoff's paper he says that Van Beneden, who examined 

 his first series of slides, concluded that in Distalpa the test-cells, 

 after originating from the follicular cells, migrated into the yolk. 

 If this be true, then the difficulties of determination of the ori- 

 gin of the test-cells would be greatly increased, from his own 

 point of view. And as in the origin of the test-cells it is not 

 conceivable that in one Ascidian we have true ova, and in an- 

 other ooblasts to replace them, when all the other processes are 

 identical, no explanation of our work being done in different 

 genera seems possible. After having gone over again and again 

 sections of ovaries prepared by many methods, I have been as 

 often forced to accept the account I have given in the first 

 part of this paper, both because no traces of nucleogemmas have 

 ever been observed, and also because I believe I have been able 

 to trace step by step the origin of the test-cells from the sur- 

 rounding follicular zone. 



The Marine Biological Laboratory, Wood's Holl, 

 July 15, 1890. 



