8 



NATURE 



[July 7, 192, 



due to the space occupied by the " accordion " folds, 

 and a diagrammatic sketch of the folded membrane 

 seen in plan with the scales removed is given in Fig. 8. 



B, 



Fig. 9.- 

 of an 



B. 



B, 



B. 



'2 ".^ "4 



-To illustrate the formation 

 " accordion " folded sheet. 



Fig. 8. — (a) Hird's-eye view of cardboard model showing the upper surface 

 and side of the folds partly extended, (b) Developed surface of card 

 showing the lines of folding. 



The positions in which the sockets for holding the 

 stems of the scales occur are shown at d in Figs. 6, 7, 8. 

 In the pupal wing the scales are closely packed like 

 the pile of a carpet, but after expansion lie close, and 



nearly parallel to the ex- 

 tended wing membrane, 

 see Figs. 4 and 5. 



The mechanical means 

 whereby the extension is 

 effected is, I believe, of 

 much the same nature as 

 pumping up a pneumatic 

 tyre, fluid being injected 

 into the nervures by the muscular action of the body ; 

 but this is a point requiring further observation. 

 So far as I know, it is only in the Lepidoptera that the 

 3 to I expansion occurs, but it seems probable that 

 the same form of stiffening by injection acts in all 

 wings supplied with nervures. 



A. Mallock. 

 April 30. 



The Formation of New Egg Cells during 

 Sexual Maturity. 



It is generally believed among mammalian embryo- 

 logists that during the life of the individual there is 

 no increase in the number of primary oocytes beyond 

 those originally laid down when the ovary was 

 formed. This idea has grown from two sources of 

 evidence — one, from the Weismannian doctrine of 

 the germ-plasm ; the other, from the fact that it is 

 difficult to find any evidence for post-natal formation 

 of new oocytes by metamorphosis of any non- 

 germinal ovarian cell. 



The problem of the origin of sex-cells in general 

 introduces two questions about which much dis- 

 cussion has taken place. The first of these questions 

 is how the first germ cells arise ; the second, whether 

 somatic cells can change into egg cells. Many, 

 accepting fully the work of Beard, that doyen of 

 embryologists, and of Woods, who showed that the 

 germ cells of certain Vertebrata originate as large 

 pale cells of the yolk-sac endoderm, at the same time 

 consider that the view that no somatic cell can 

 metamorphose into a germ-cell needs more evidence 

 than the description of germ-cell migration. Apart 

 from this important question, some zoologists believe 

 that no accession of new egg cells takes place during 

 the post-natal life of any craniate vertebrate, but the 



evidence produced by Bouin, Braun, Ludwig, and the 

 writer would seem to be conclusive for fish, amphi- 

 bians, and reptiles. 



The attached photo-micrograph (Fig. i) of the 

 adult frog ovary shows a large ovarian tag containing 

 germ cells in all stages, and it is indisputable that in 

 vertebrates below the mammals seasonal accessions 

 of new germ cells take place. 



So far as the mammals are concerned no observer 

 within recent years has attacked the problem, but 

 Edgar Allen in the American Journal of Anatomy, 

 vol. 31, No. 5, has now published a paper in which 

 he claims that a cyclical proliferation of the germinal 

 epithelium gives rise to a new addition of young 

 oocytes in the cortex of the adult ovary of Mus at 

 each normal oestrus period. This new paper appears 

 to me to contain the results of much careful work, 

 and it upholds the views expressed by the Waldeyer 

 school of embryologists. 



So far as the mammal is concerned, it may be taken 

 that since the necessity for large numbers of fresh 

 proliferated germ cells is usually absent, these do not 



Fig. I. — Photomicrograph of adult ovary of Rana. At X, X, are leptotene 

 and pachytene stages of oogenesis, as well as large numbers of later 

 oocytes. At Oj is a part of the rest of the ovary with large eggs. 



generally occur in those forms which produce few 

 young. The writer, through the kindness of Prof. 

 J. P. Hill, has lately examined several ovaries of 

 Ornithorhynchus without finding any signs of oogonia 

 in the adult : the material was not extensive enough, 

 in the light of Edgar Allen's work, to pronounce a 

 definite verdict, but I believe Ornithorhynchus does 

 not produce litters of young like the rodent. It is 

 worthy of note that Allen's descriptions of photo- 

 micrographs correspond to the descriptions and 

 figures already given by the present writer for Rana 

 and Bufo. Allen's Plate 5 is very striking evidence ; 

 he has, moreover, demonstrated completely the 

 cyclical mitotic divisions and activity in the germinal 

 epithelium of Mus. 



The opponents of the germinal epithelial theory 

 will naturally say that Allen's cells are derivatives of 

 the migrated primitive germ cells ; but unless some 

 obvious difference can be shown to exist between the 

 germinal epithelial cells and the forerunners of the 

 cells described by Allen, we are justified in assuming 

 that the new egg cells are derived from metamor- 

 phosed epithelial cells, and certainly from cells which 

 have lost their individuality in the formation of the 

 so-called somatic part of the ovary. 



The statement, often made, that only primordial 

 germ cells can produce gametes, and that the metamor- 

 phosis of epithelial cells into germ cells does not take 

 place, needs also the assumption that the potential- 



NO. 2801, VOL. I 12] 



