No. 2.] TEST-CELLS OF ASCIDIANS. 20I 



these ingrowths become constricted off to form the test-cells. 

 After separation from the follicular zone, the test-cells multiply 

 at the periphery of the c^^'g, and form each clusters of four or 

 five cells closely connected. 



In Fig. 17 is shown a section through the periphery of the 

 egg, taken at a time when the test-cells were entirely within 

 the follicular zone. Three such cells are seen in the figure, and 

 it will be also noticed that at the same time the peripheral zone 

 of protoplasm of the egg becomes extremely narrow, and at the 

 time of its disappearance a membrane appears around the o.^'g 

 between the follicular cells and the yolk, and enclosed within 

 this membrane are the test -cells. Most probably this peripheral 

 protoplasm either secretes the membrane before its disappear- 

 ance, or becomes converted into it. 



In Fig. 18 is drawn a part of the periphery of an egg of 

 Clavellijia when the o.^^ is nearly mature. The test-cells are 

 seen lying in clusters of from one to a dozen at the periphery of 

 the yolk and just within the o.g'g membrane. Outside of the 

 membrane the columnar follicular cells form a layer completely 

 investing the ^g'g, and beyond the follicular cells is an imper- 

 fect layer formed of scattered cells which seem to be derived 

 from the inner substance of the ovary, although I have not care- 

 fully traced their origin. 



The account I gave of the formation of the test-cells of Clavel- 

 lina in my preliminary note is not quite clear nor entirely free 

 from error. This is due to my not distinctly recognizing that 

 the peripheral zone of protoplasm of the &gg belonged entirely 

 to the ovum ; but I hope the preceding account will make clear 

 the true position of the o.^^ membrane. 



It seems useless to give an extended criticism of the results 

 and theory of Davidoff. Our views are diametrically opposed, 

 and it seems to me irreconcilable. 



My own results agree essentially with those of Van Beneden 

 and Julin, and must stand or fall with theirs. Those who care 

 for a fuller knowledge of Davidoff's position can refer to his 

 paper {in the Mittheilungen aus der Zoologischen Station zu 

 Neapel. Neunter Band. I. Heft. 1889). I will give here very 

 briefly an outline of his views, so that I may contrast them with 

 the preceding account. 



In very young eggs (much younger than those in which I 



