ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 539 



old lutein cells, or the cells of an atresic follicle, and there is no evidence 

 that any of the three last are at any time transformed into interstitial 

 cells. Such cells are present in the pouch young of T. vulpecula before 

 they could have been derived from any of these sources. Interstitial 

 tissue is to be regarded as a tissue siii generis, although it is possible 

 that it may originate from stroma cells at a very early stage. The 

 tissue is irregularly distributed in the various species of Marsupials, 

 and it is worthy of note that it is present in all the Diprotodontia and 

 absent in the Polyprotodoutia so far examined. It may be present only 

 as a few small groups of cells, or in such quantity as to form by far the 

 largest part of the bulk of the ovary, excluding corpora lutea, e.g. in 

 P. penicillata. The tissue has a typical glandular appearance, but, as 

 no satisfactory account of its function has yet been put forward,, it 

 seems preferable to retain the term interstitial tissue or cells rather than 

 to call it a gland. 



Theory of Sex.* — E. A. Goeldi discusses the nature and origin, 

 determination and significance of sex in the light of modern investi- 

 gations. Two general conclusions are emphasized. The first is that 

 differentiation of the sexes has to do with the conservation and rejuve- 

 nescence of species. The second is that optimum conditions favour the 

 production of females rather than males, and that hard condiiions of 

 existence favour the production and predominance of males. The paper 

 has graphic illustrations. 



Inhibition of Development of Artificial Parasitism.! — G. A. Belo- 

 golowy has made the experiment of putting eggs of frogs into the 

 body-cavity or into the tissues of frogs (Felobates). He implanted fifty 

 eggs of Felobates, and after autopsies four months afterwards he found 

 eighteen of them again. An account is given of their appearance — 

 usually densely black spheres, from 0*5 to 4 mm. in diameter, flattened 

 on the side next the organ to which they were attached. They were 

 multicellular and showed peculiar tissues, for the most part embryonic. 



In the free stages the spheres were somewhat uniform masses of 

 pigmented tissue, with a connective tissue envelope, which sometimes 

 grew inwards. In fixed stages capillary vessels appeared. Capsules 

 inside the spheres contained peculiar free elements, which the author 

 calls X-cells. Later on there was a multiplication of these, and a mass- 

 of embryonic tissue resulted. But besides this there was often a 

 " mimicry " of adjacent host-tissue. 



The " mimicry " is not due to adjacent host-tissue growing in. The 

 resemblance is usually to connective tissue, rarely to hepatic tissue. 

 Some of the spheres form colonies in the liody-cavity. On the whole, 

 the spheres remain at a blastula level. There is no exception to the 

 occurrence of the X-cells. A description is given of primitive pigmented 

 tissue, several kinds of connective tissue and other approximations to 

 some development on the part of the spheres. The differences in the 



* MT. Nat. Ges. Bern, 1916, pp. 140-203 (12 figs.). 



t Nouv. Mem. Soc. Imp. Nat. Moscow, xviii. (1914) p. 1-50 (3 pla.). 



2 p 2 



