October 22, 1908] 



NA TURE 



649 



even certainty, that the approach would not necessarily 

 be one-sided, but might be convergent, eacli form in some 

 respects advancing to meet tlie other. This suggestion, 

 however, was never developed by Fritz Miiller, for although 

 he mentioned a few instances in support of his view, he 

 did not attempt to trace the supposed mutually mimetic 

 process in any detail. Dr. Dixey showed that there is 

 much evidence that such reciprocal approach, or inter- 

 change of obvious characters — for which the term diapose- 

 matism has been proposed by Prof. Poulton — does actually 

 occur, and he exhibited some cases of mimicry the peculiar 

 features of which are difficult to explain on any other 

 hypothesis. 



Prof. E. B. Poulton exhibited and described specimens 

 illustrating mimicry in the butterflies of North America, 

 and then proceeded to give an account of some recent 

 investigations upon the African swallowtail, Papilio 

 dardanus (inerope), as an example of mimicry. 



Mr. J. C. Moulton exhibited four groups of South 

 American butterflies to illustrate the Miillerian theory of 

 mimicry. In the Venezuelan group the general warning 

 pattern consists of a chestnut background relieved by 

 transverse black bars and yellow apical markings on the 

 forewings. In the other groups the pattern is modified 

 in various ways according to the environment ; for 

 example, in the Trinidad group yellow takes the place 

 of the chestnut background, while the group from Ega, on 

 the Upper Amazon, presents a more mottled and richer 

 brown appearance, and in the fourth group, from Ecuador 

 and Peru, the bars and mottled markings have given 

 place to a dark background relieved only by a broad, 

 oblique chestnut band. 



The Development of Liiiorina. 



Mr. W. M. Tattersall briefly described the development 

 of several species of Littorina. The eggs of L. littorea, 

 each enclosed in a hat-shaped capsule, are laid freely on 

 the shore, and not aggregated together in a gelatinous 

 mass. The larva leaves the egg as a trochosphere, and 

 passes through a veliger stage to the adult. L. littorea 

 lives low down on the shore among Laminaria and Fuctis 

 serratus. L. obtusata lives in a higher region of the 

 shore, in the zone of Fitciis vesicidosits ; its larva leaves 

 the egg as a veliger. L. rudis and L. neritoides, both of 

 which live near high-water mark, are viviparous. Thus 

 within the limits of a single genus there are presented 

 three stages in the evolution of land Mollusca from marine 

 forms, showing specialisation in reproduction and gradual 

 abbreviation and final suppression of larval stages, corre- 

 lated with successive stages of specialisation of habitat. 



Gastrulalion in Amphioxus. 

 Prof. E. W. MacBride pointed out that two theories 

 have been advanced as to the mode in which the germinal 

 layers are formed in .Amphioxus : — (i) that the invagina- 

 tion is a simple process, and that the whole of the in- 

 vaginated layer is endoderm from which notochord and 

 mesoderm take their origin by a process of folding (as 

 held by Kowalevsky, Hatschek, Samassa, and MacBride) : 

 (2) that the invagination is a double process; on the ventral 

 side of the blastopore the cells are true endoderm, while 

 on the dorsal side they are ectoderm (as held by Cerfon- 

 taino), and that the ectodermic roof of the archenteron 

 becomes used up in the formation of the notochord and 

 mesoderm, which are cut out of the wall of the archenteron 

 by the upgrowth of the true endoderm cells at the sides. 

 Prof. MacBride's observations lead him to conclude that 

 all the intucked cells are endoderm, that the mesoderm 

 originates from a dorso-lateral fold of the endodermic 

 wall, which becomes cut into anterior and posterior halves 

 by the growth of a septum ; both halves of the fold remain 

 open into the gut. The front half becomes eventually 

 closed off, and corresponds to the mandibular head-cavity 

 of other vertebrate embryos and to the collar-cavity of 

 Balanoglossus. The posterior division, corresponding to 

 the lateral plate of mesoderm of other vertebrate embryos 

 and to the trunk cavities of Balanosriossus, retains its 

 connection with the gut for a longer time ; from its front 

 end the somites of the body are cut off. The head 

 cavities arise still l.-ter as a single median invagination 



NO. 2034, '^OL. 78] 



of the anterior gut wall, which, before it separates from 

 the gut, begins to be divided into right and left halves. 

 The speaker concluded that, with some slight modifications, 

 the simple view of the development of Amphioxus held by 

 Kowalevsky and Hatschek was to be maintained. 



The Early Development of Dasyurus. 



Prof. J. P. Hill gave an account of the early develop- 

 ment of the native cat (of southern Australia) — Dasyurus 

 viverrinus. The uterine ovum is of large size as compared 

 with the ova of Eutheria, is enclosed in a shell membrane, 

 and exhibits a marked polarity, its lower pole consisting 

 of dense, finely granular cytoplasm in which the pronuclei 

 are situated, and its upper pole of a delicate reticulum 

 with fluid-filled meshes. Prior to cleavage this latter por- 

 tion of the ovum is separated off and takes no further part 

 in development. The fluid material in this non-formative 

 portion of the ovum is to be regarded as the product of 

 an abortive attempt at the formation of a solid yolk-mass. 

 By its elimination the potentially yollc-laden telolecithal 

 ovum becomes converted into a secondary homolecithal 

 homoblastic one. The first three cleavage planes are 

 meridional ; the resulting eight blastomeres are of equal 

 size, and form an equatorial ring. The ensuing divisions 

 (fourth cleavage) are parallel to the equator and are un- 

 equal, each of the eight blastomeres becoming divided 

 into an upper, smaller, and clearer cell with relatively 

 little deutoplasm, and a lower, larger, and denser cell 

 with well-marked deutoplasmic contents. A sixteen-celled 

 stage is thus produced in which the cells are arranged in 

 two superimposed rings, each of eight cells. The de- 

 scendants of these two cell-rings gradually spread towards 

 opposite poles in "contact with the thickened shell mem- 

 brane, and constitute the cellular wall of the blastocyst, 

 which is unilaminar, and remains so until the vesicle 

 attains a diameter of 4-5 mm. The upper cell-ring is 

 regarded as furnishing the formative (embryonal) portion 

 of the vesicle wall — the homologue of the embryonal knot 

 of the eutherian blastocyst — from which are derived the 

 embryonal ectoderm and the entire endoderm, while the 

 lower ring gives rise to the extra-embryonal portion of the 

 vesicle wall, the trophoblastic ectoderm. The markedly 

 different mode of formation of the blastocyst in the 

 Eutheria was regarded as correlated with the complete 

 loss of the shell membrane in the course of their phylo- 

 geny. 



The Wild Ancestors of Domestic Horses. 



Prof. J. C. Ewart gave an account of the wild ancestors 

 of domestic horses, dealing particularly with (i) Equus 

 sivalensis of northern India, a long-limbed form with the 

 face strongly bent downwards ; (2) E. prcjvalskii, the 

 horse which still lives in a wild state in Mongolia ; 

 (3) E. robiistiis, the remains of which occur at the Palaeo- 

 lithic settlement of Solutr^, north of Lyons; and (4) E. 

 gracilis, of the Auvergne and other French Pleistocene 

 deposits, which seems to have given rise to E. caballus 

 libycus (Ridgeway) of north -Africa and to the Celtic pony 

 (£. caballus celticus) of north-western Europe. Dr. Scharff 

 mentioned that the horse remains found in the Irish 

 crannogs, bogs, and caves bore out Prof. Ewart 's view 

 that a wild horse formerly existed in Ireland, of which 

 the present-day Connemara pony seems to be the direct 

 descendant. 



Feeding Habits of British Birds. 

 Mr. C. Gordon Hewitt advocated the institution of an 

 inquiry into the feeding habits of British birds, and urged 

 that, in order to obtain as accurate a conception as possible 

 of the economic status of any species of bird, it was neces- 

 sary to examine and record the contents of the crop and 

 stomach of a large number of individuals killed, not only 

 in different months of the year, but also in different locali- 

 ties. .Such evidence would provide the only safe guide to 

 the protection of wild birds. 



Dr. C. J. Patten gave an account of the migratory 

 movements of certain shore birds, especially the sanderling 

 and turnstone, as observed on the Dublin coast, and showed 

 skins illustrating the phases of plumage changes according 

 to sex, a£?e, and season. 



Prof. W. .A. Herdman gave some natural-history notes 



