EMBRYOLOGTCAL METHODS. 341 



embryonal membranes have been formed, the shell may be 

 scraped away at some spot and there treated with picro- 

 sulphuric acid until a small hole is formed ; then by working 

 away from this spot, by means of scraping and dropping 

 acid on to it, the whole of the shell may be removed. 



WILL (Zool. Jahrb., AUh. Morph., vi, 1892, p. 8) opens 

 ova of Platydactylus in the fixative (chiefly chromic acid, or 

 chromo-aceto-osmic acid with very little osmic acid) and 

 hardens the embryos on the yolk ; so also for Cistudo and 

 Lacerta (1893 and 1895). MEHNERT (Anat. Anz., xi, 1895, 

 p. 257) does not approve of these methods; for his own see 

 Morph. Arb. Kchwalbe, i, 1891, p. 370. 



STEAHL (Arch. Anat. Phys., Anat. Abth., 1881, p. 123) 

 uses picro-sulphuric acid for Lacerta. 



KUPFFER (op. cit., 1882, p. 4) removes ova of Lacerta, 

 Emys, Coluber, etc., from the oviduct, opens them under O'l 

 per cent, osmic acid, removes as much of the albumen as 

 possible, brings the yolk for twenty -four hours into chromic 

 acid of 1 : 300 strength, then excises the blastoderm, washes, 

 puts it for three hours into a mixture of glycerin, alcohol, 

 and water in equal parts, and lastly into 90 per cent, 

 alcohol. 



See also previoiLs editions. 



Amphibia. 



600. Preliminary. In order to prepare ova for section- 

 cutting, it is essential to begin by removing their thick 

 coats of albumen. This may be done by putting them for 

 two or three days into 1 per cent, solution of chromic acid, 

 and shaking well ; but ova thus treated are very brittle, 

 and do not afford good sections. A better method is that 

 described by WHITMAN (Amer. Natural., xxii, 1888, p. 857), 

 and by BLOCHMANN (Zool. Anz., 1889, p. 269). WHITMAN 

 puts the fixed eggs into a 10 per cent, solution of sodium 

 hypochlorite diluted with 5 to 6 volumes of water, and 

 leaves them there till they can be shaken free, which 

 happens (for Necturus] in a few minutes. BLOCHMANN takes 

 eau de Javelle (potassium hypochlorite), and dilutes it with 

 3 to 4 volumes of water, and agitates the eggs previously 



