84 bulletin: museum of comparative zoology. 



and the large subsequent increase in the numbers of the latter must be 

 due to their division. There is no reason to doubt that they divide 

 by mitosis, for at this stage they have a perfectly healthly appearance, 

 and in fact Davidoff (pp. 131-132) has described and figured mitoses of 

 both test and follicle cells in D. magnilarva. 



c. Degeneration of the Test Cells* 



The degeneration of the test cells in Distaplia is a much simpler 

 process than that occurring in some ascidians, consisting, as Davidoff 

 has shown, in the vacuolation of the cytoplasm. The cellular nature 

 of the product is always evident, as the distinctive character of the 

 nucleus is never lost, though this organ is forced into a peripheral 

 position and changes its appearance slightly. This degenerative vacuo- 

 lation begins in our species at about the same time as the yolk for- 

 mation (Plate 4, Fig. 28) and is completed (Plate 5, Fig. 30) before 

 the last changes in the yolk occur. Figure 29 (Plate 4) shows a 

 few test cells in which the process is just beginning. At this stage the 

 test cells are often numerous enough to form a double row about the 

 periphery of the ovum (Fig. 28), but sometimes they form only a 

 single row. As the ovum passes through the oviduct into the pouch, 

 it seems to contract somewhat, forcing the test cells out, and the yolk 

 bodies between which they were formerly situated become arranged so 

 as to form a smooth surface at the periphery of the ovum (Plate 5, 

 Fig. 31). However, in spite of this tendency, there is occasionally 

 enough pressure exerted upon the test cells to force them partly into 

 the yolk again. 



Up to this time the nucleus has changed little if any, but from now 

 on further changes in the test cells are almost entirely confined to this 

 organ. During the growth of the embryo, the nucleus loses most of its 

 chromatin, its membrane only staining, and becomes compressed be- 

 tween the cell membrane and adjacent vacuoles (Plate 5, Figs. 32, 34). 



d. Fate and Function of the Test Cells. 



Among the early investigators the belief that the test of the larval 

 ascidian was formed by the test cells was universal, and it was on this 

 account that Kupffer ('70, p. 122) applied this name to them. In 1872, 

 however, Herting ('72) showed that the test first appeared as a thin 

 layer next to the ectoderm of the larva and entirely within the so-called 

 test cells, which took no part in the process. This view has been gen- 



