460 NOTES AND MEMORANDA. 



said that liis attention had been called a number of years ago to a 

 similar black appearance on the brick walls and granite work of houses 

 in narrow shaded streets, especially in the vicinity of the Delaware 

 river. Noticing a similar blackness on the bricks above the windows 

 of a brewery, from which there was a constant escape of watery vapour, 

 in a more central portion of the city, he was led to expect that it 

 was of a vegetable nature. On examination, the black mildew proved 

 to be an alga closely allied to what he supposed to be the Protococcus 

 viridis, which gives the bright green colour to the trunks of trees, 

 fences, and walls, mostly on the more shady and northern side. It 

 probably may be the same plant in a different state, but, until proved 

 to be so, may be distinguished by the name of Protococcus luguhris. It 

 consists of minute round or oval cells, from 0-006 to 0*009 mm. in 

 diameter, isolated or in pairs, or in groujis of four, the result of 

 division ; or it occurs in short irregular chains of four or more cells 

 up to a dozen, occasionally with a lateral offset of two or more cells. 

 By transmitted light the cells appear of a brownish or olive-brownish 

 hue. In mass, to the naked eye, the alga appears as an intensely black 

 powder.* 



MICEOSCOPY. 



Employment of Wet Collodion for Microscopic Sections. — 

 M. Mathias Duval points out t the difficulty of finding any body which 

 would firmly hold delicate objects, in which there are a large amount of 

 hollows and cavities, such, for example, as embryonic tissues ; it is 

 obvious that the best substance would be one, which though solid is 

 not friable, and which at the same time is homogeneous ; these con- 

 ditions are not satisfied by the ordinary imbedding mixtures, such as 

 gelatine, wax and oil, or soapy boilies ; one that has been largely used is 

 gum solidified by the action of alcohol, and this has been recommended 

 by Dr. Klein ; in the directions appended to their ' Treatise on 

 Embryology' (of the Chick), Foster and Balfour expressly state 

 that they do not recommend it for the study with which they are 

 there particularly engaged, nor does the experience of other embryo- 

 logists seem to do otherwise than confirm their opinion. Nor, again, 

 do the methods ordinarily in use allow of the advantages which Avould 

 be gained by the use of a transparent imbedding substance. 



Already used in its dry state for certain observations, collodion 

 has been found to have much to recommend it, but it is too hard for 

 delicate bodies; when, however, a small quantity is treated with 

 alcohol at 36°, it is foimd to retain its volume, while presenting a 

 large amount of consistency, elasticity, or tiansparency. Having 

 used the substance for six months, M. Duval now feels justified in 

 recommending it to the attention of students ; the embryos to be 

 examined are first hardened by osmic acid, alcohol, or some other 

 method, are stained with carmine, and then placed in alcohol ; they 

 are placed for a few minutes in ether, and are then removed to 

 the liquid collodion, in which they remain for a period varying from 



* ' Proc. Acad. Nat. Sci. Phila.' (1S7S) p. 331. 

 t ' Journ. Anat. et Phys.' (Robin), xv. (1879) 183. 



