184 PROGRESS OF MICROSCOPICAL SCIENCE, 



fragments of wliicli it is composed are exceedingly minute ; but 

 granules of quartz of an appreciable size, and affording colours with 

 polarized ligbt, are disseminated throughout the fine-grained matrix. 

 A greenish tint is also imparted to it by chlorite ; and it is minutely 

 divided by a system of markings made up of pairs of nearly parallel 

 lines, each about yJ^gth of an inch in length and ^^^Qp th of an inch 

 apart. These cross each other so thickly as to form a kind of close 

 network, and give no colours by polarized light ; they were at first 

 taken for amorphozoa ; but his friend Mr. E. Etheridge, who kindly 

 examined them for him, is of opinion that they are " certainly not 

 organic." In regard to the " Koofing-slate, Delabole," the author, 

 unable to arrive at a decisive opinion, sent a specimen to Mr. Sorby, 

 F.K.S., who says : — " The dark crystals in the slate from Delabole are 

 similar to what I have seen in many others. As far as I can make out, 

 they are imperfect hexagonal plates ; and since the general character 

 clearly shows that they must at all events contain much iron, it seems 

 very probable that they may be more or less altered specular iron. 

 This, of course, would well agree with the fact of its containing titanic 

 acid." The author gives several other examples ; but for further 

 information we must refer our readers to the journal in which the 

 paper originally appeared. 



Species of Atax Parasitic on Fresh-ivater Mussels. — M. Emil Bessels 

 has written a paper on the above subject in the ' Wiirtembergische 

 Naturwissenschaftliche Jahrrshefte ' for 1867, which has been trans- 

 lated by Mr. Dallas into the ' Annals of Natural History ' for January. 

 The paper is of some little length, and will well repay perusal. His 

 observations have been especially made on the development of Atax 

 ypsilophorus, and as far as originality is concerned refer to the forma- 

 tion of the blastoderm. How long, for instance, after the deposition 

 of the eggs the blastoderm makes its appearance no one can say with 

 certainty, inasmuch as the deposition itself cannot be observed. In 

 eggs which were taken from the branchiae of the Unio or Anodonta, 

 and apparently had undergone no change after deposition, he usually 

 detected the first traces of the blastoderm in from two to three days. 

 It is formed insularly, as may be easily proved by opening an egg 

 carefully in a solution of 1 per cent, of bichromate of potash. It is 

 impossible to ascertain the process of formation by the direct observa- 

 tion of the uninjured egg, on account of the dark colour of the yelk. 

 After the blastoderm has grown round the whole of the yelk, the em- 

 bryonal envelope which Claparede describes as the deutovum separates 

 from it. This is produced in exactly the same manner as the larval 

 membrane of the Crustacea, as observed by Van Beneden and myself 

 in various species of Gammarus* Claparede was at first inclined to 

 regard this envelope as the homologue of the : structui'e which in 

 insects has received the unfortunate name of the " amnion ;" but he 

 soon gave up this comparison. M. Bessels, on the other hand, 



* E. van Beneden and E. Eessels, " Eesiime d'uu Memoire sur le Mode de For- 

 mation du Blastoderme dans quelqucs groups dc CrustaceV' 'Bull. Acad. Roy. 

 Belg.,' 2" scr., xxv., p. 443. 



