28 Transactions of the Royal Canadian Institute 



area which is a miniature of the entire plate. It is quite evident that 

 these are Hnes of growth, which represent the margin of the plate at 

 successive stages in its growth. We interpret these lines as "checks' 

 in the growth of the test, that is that growth is periodic and that its 

 renewal after a period of quiescence produces a definite line. The princi- 

 pal periodicity in conditions is that of the seasons, winter alternacing 

 with summer. In boreal or arctic regions the high temperature of 

 summer is more favourable for activity, reproduction and growth in 

 the majority of animals than is the low temperature of winter. It has 

 been observed that Ascidians reproduce and grow during the summer 

 and that during the winter season some species are in a reduced or 

 quiescent state, with more or less of the body sloughed away to be 

 regenerated in the spring. We are, therefore, justified in assuming that 

 the lines of growth in the plates of Chelyosoma represent winter periods 

 with a lessening or a stoppage of growth, and all the available facts 

 support this assumption. The retention of the winter border of the 

 plate as a line is perhaps to be explained as the result of the hardening 

 of the outer part of the test, which would mean that, when growth is 

 interrupted for a time long enough to permit of the test at the edge of 

 the plate hardening, the new growth is unable to lift up the turned-in 

 edge of the plate and a groove remains between the old test and the 

 new. The hardening of the test is due in part at least to the formation 

 of a special outer layer, the yellow layer as it is called by Bancroft (1898, 

 P- 317); (2) who has described it for C. productum. It is comparatively 

 thin and "is slightly harder, stififer, and more brittle than the tunicin 

 layer". Bancroft describes its origin as a secretion from mesodermal 

 bladder cells, which move to the surface of the test and finally degenerate. 

 In this species these cells are quite regularly arranged at the surface and 

 each is surrounded by a more or less d<efinite zone, which is apparently 

 the product of its activity. Bancroft has described the formation of a 

 deposit of the yellow substance just inside and just outside the cell 

 membrane of the bladder cell and states that "of those nearest the surface 

 nothing can be seen but a small vesicle containing some refractile ma- 

 teiial in which usually not even the remains of a nucleus can be made 

 out". In the newer portions of the yellow layer, these cells are larger, 

 and the zone of yellow substance surrounding each one is more distinct 

 than is the case in the older portions. If the lines of growth are to be 

 used in calculating the growth of the plate or of the, individual in suc- 

 cessive years, it is necessary that there be no interstitial growth in the 

 old portion of plate. 



(^) The Anatomy of Chelyosoma productum Stimpson. Proc. Calif. Acad. Sc., Ser. 3, 

 Zool. Vol. I, pp. 309-332. 



