BANCROFT : OVOGENESIS IN DISTAPLIA OCCIDENTALTS. 87 



test by the follicular epithelium (Plate 5, Fig. 32, tst.). In spite of this 

 fact, however, the outer layer of the test matrix is quite thin, and under 

 it the beginnings of the first lacunae (lac.) are seen. Occasionally we see 

 cells (Fig. 32, cl. ms'drm.) within the lacunae, but these have none of the 

 characters of the test cell, whereas they do resemble some of the mesen- 

 chyme elements very closely. It seems hardly possible, then, that the 

 test cells can have had anything to do with forming this structure. 

 Moreover, in following the series through ten sections, 6 /j. thick, not 

 another cell is found within the test of this region. It is seen therefore 

 that the first tunicin and some of the first lacunas are formed from the 

 ectoderm, not only without the intervention of the test cells, but also 

 without the aid of any cells lying within the test substance. 



With further increase in the thickness of the test the lacunae increase 

 in size, and many mesoderm cells wander into them, and into the matrix 

 between them (Plate 5, Figs. 33, 34, cl. msdrm.). These cells often 

 flatten out against the lacunar wall, and line it for a variable distance, 

 but in the test that is about as thick as the test cells ; this condition is 

 rarer than at a later period, when there are several layers of lacunae. 



During the whole of the development, up to the time when the larva 

 is extruded, the test cells are often closely pressed against the test ; but 

 there appears to be no marked decrease in the number of these ele- 

 ments, though, of course, on account of the greater size of the embryo, 

 in the later stages they become more widely separated. In this species, 

 then, the evidence points strongly to the conclusion that the test cells 

 have nothing whatever to do with the formation of the test, but that 

 the matrix of the latter, together with its lacunae, is formed from the 

 ectoderm, and that its cells are mesodermic elements that have subse- 

 quently wandered in. 



What, then, is the function of the test cells in Distaplia, if they have 

 nothing to do with the formation of the test, and what is the function 

 of these cells in general] Many authors are not satisfied with their 

 role in nourishing the ovum, but think that they must be functionally 

 active as long as they remain near the growing embryo. Thus, Salensky 

 ('92, pp. 109-110) says: " Es wurde vielmehr still schweigend ange- 

 nommen, die Testazellen hatten keine Bedeutung bei der Bildung des 

 Embryos der Ascidien. Ein solcher Schluss war fur mich von Anfang 

 an nicht ganz befriedigend." Floderus, after a careful discussion of the 

 literature, so far fails to realize the true nature of these cells that he 

 says (p. 244) : " Ich halte es fur wahrscheinlich, dass die Testazellen 

 eine Art von rudimentaren Bildungen sind, welche nunmehr eiue un- 



