ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 347 



Feeding Experiments on Tadpoles.*—.). F. Gudernatsch finds 

 that there is a certain constant minimum size which tadpoles must 

 reach before the final metamorphosis can begin. There is, on the other 

 hand, a constant assimilation of food and a gradual increase in body size- 

 up to a constant maximum, beyond which a normal animal may not ]»as> 

 without the beginning of the final differentiation and metamorphosis. 



In thyroid-fed tadpoles there is differentiation without growth. In 

 thymus-fed (and spleen-fed) tadpoles there is growth without differen- 

 tiation. There are thus two distinct factors — the factor of growth and 

 the factor of differentiation. The two naturally work simultaneously, 

 but differentiation is not the result of growth, nor is growth necessarily 

 followed by differentiation. The thyroid must possess a capacity for 

 stimulating differentiation which other foods do not possess ; the thymus 

 (or spleen) has a quality that suppresses differentiation. Thus, the 

 thyroid and thymus must produce, or at least contain, agents which, 

 when passed into developing organs, in the one case stimulate, in the 

 other suppress, differentiation. The production of such substances to 

 be thrown into the circulation characterizes the thyroid and thymus as 

 glands with a positive internal secretion. 



Weight of Thymus in Rat according to Age.f — Shinkishi Hatai 

 finds that the thymus in the albino rat (Mus norvegicus) shows great 

 variability in weight. The variability is greatest at the period of rapid 

 growth of the body, and becomes less as the growth of the body is slowed. 

 The greater variability during the first six weeks may be attributed to 

 the greater variability of the body weight, for the weight of the thymus 

 at this period varies not only with age, but also with the body weight, 

 while in the older rats this is not the case. 



Interstitial Cells of Mammalian Ovary. J — B. F. Kingsbury has 

 studied these elements in the cat. They are modified stroma cells, and 

 hence of connective tissue origin. Their origin in the adult as an 

 hypertrophy of theca cells during atresia folliculi is confirmed. In the 

 foetus and in the new-born, and immature kitten, they appear associated 

 with the irregular so-called medullary cords and follicle formations of 

 these periods. Free lipoid cells appear in the indifferent cells of the 

 atretic follicles, medullary cords and irregular medullary formations in 

 parts not associated with the ova. The development of interstitial cells 

 appears to he correlated with the activity of the indifferent or follicle 

 cells in the absence of germ-cells. The suggestion is strong that an 

 element of degeneration is involved. The zone in which interstitial 

 cells occur conforms to the centrifugal march of differential growth in 

 the ovary. Xo morphological value is attached to the distinction 

 between a foetal (or presexual) and an adult grouping of the cells. Xo 

 evidence is found for regarding the interstitial cells as constituting 

 morphologically an intra-ovarian gland. The recognition of them as a 

 gland of ovarian secretion is regarded as without sufficient evidence. 



* Amer. Jouru. Aiiat., xv. (1911) pp. 431-78 (2 pis.). 

 + Auier. Journ. Anat.* xvi. (1914) pp. 251-7 (1 chart). 

 X Amer. Journ. Anat., xvi. (1914) pp. 59-95 (3 pis.). 



