302 EMBRYOLOGIOAL METHODS. 



and several pelagic forms, and with Chironomus, Asellus 

 aquaticus, Ascidians, Planorbis, many Ccelenterata, &c. I 

 advise the student to carefully draw the different stages so 

 observed, for such drawings are most important aids to the 

 study of the same stages by the section method. 



Some ova of Insecta and Arachnida which are completely 

 opaque under normal conditions become transparent if they 

 are placed in a drop of oil; if care be taken to let their 

 surface be simply impregnated with the oil, the normal course 

 of development is not interfered with (BALBIANI). 



586. Preparation of Sections. Osmic acid, employed either 

 alone or in combination with other reagents, is an excellent 

 fixing agent for small embryos, but not at all a good one for 

 large ones. It causes cellular elements to shrink somewhat, 

 and therefore brings out very clearly the slits that separate 

 germinal layers, and any channels or other cavities that may 

 be in course of formation. 



In virtue of its property of blackening fatty matters, 

 myelin amongst them, it is of service in the study of the 

 development of the nervous system. 



Chromic acid is indispensable for the study of the external 

 forms of embryos ; it brings out elevations and depressions 

 clearly, and preserves admirably the mutual relations of the 

 parts ; but it does not always preserve the forms of cells 

 faithfully, and is a hindrance to staining in the mass. 



Picric liquids have an action which is the opposite of that 

 of osmic acid; they cause cellular elements to swell some- 

 what, and thus have a tendency to obliterate spaces that 

 may exist in the tissues. But notwithstanding this defect, 

 the picric compounds, and especially Kleinenberg's picro- 

 sulphuric acid, are amongst the best of embryological fixing 

 agents. 



For imbedding, the celloidin-chloroform method of Viallanes 

 gives excellent results, and so does paraffin. This latter is 

 preferable in so far as it lends itself better to the rapid pro- 

 duction of series of sections, and allows of the use of the 

 Cambridge Rocking Microtome, or the Minot, which is per- 

 haps the microtome par excellence of the embryologist. 



As to staining, my eminent fellow- worker, Dr. Henneguy, 

 writing the chapter on embryological methods for the French 



