288 riiAlTER XXV. 



586. Superficial Examination. --The development of some 

 animals, particularly some invertebrates, may be to a certain 

 extent followed by observations of the living ova under the 

 microscope. This may usefully be done in the case of 

 various Teleosteans, such as the Stickleback, the Perch, 

 Macropodus, and several pelagic forms, and with Chironomus, 

 Aselluts aqiiaticus, Ascidians, Planorbis, many Ccelenterata, 

 etc. 



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 (BALJBIANI). 



587. Fixation. 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 bulk. 



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. 



SCHEIDDE (Zeit. wiss. Milt., xxvii, 1910, p. 362) finds Orth's 

 " Formol-Miiller } in general the best fixative. Fix for not 

 more than 24 hours, and pass through graded alcohols (20 

 minutes in each) into absolute (1 to 2 hours), cedar oil, 

 xylol, and paraffin. 



