360 EMBRYOLOGY 



the males in the same way. (It may occasionally be necessary, 

 as in the case of the Stickleback, to kill the male, and dissect out 

 the testes and tease them.) The spermatozoa of fish, especially 

 those of the Salmonida?, lose their vitality very rapidly in water ; 

 it is therefore advisable to add the milt immediately to the 

 spawned ova, then add a little water, and after a few minutes put 

 the whole into a suitable hatching apparatus with running water. 

 Artificial fecundation of Invertebrates is easily performed in 

 a similar way. For methods of artificial Parthenogenesis see 

 Harvey, Biol. Bull. Wood's Hole, 1910, p. 269. 



797. 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, Asellus aquaticus, 

 Ascidians, Planorbis, many Coelenterata, 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 develop- 

 ment is not interfered with (Balbiani). 



798. Fixation. Osmic acid, employed either alone or in com- 

 bination 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 faith- 

 fully, 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 somewhat, 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. 



ScHRiDDE {Zeit. wiss. Mik., xxvii, 1910, p. 362) finds Orth's 

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

 than twenty-four hours, and pass through graded alcohols (twenty 

 minutes in each) into absolute (one to two hours), cedar oil, xylol, 

 and paraffin. 



