316 EMBRYOLOGICAL METHODS. 



ture of the Hertwigs ( 542), and as soon as the blastoderm 

 turns white and opaque removes it under dilute glycerin. 

 Treatment with liquid of Perenyi is recommended for surface 

 views of cleavage. 



614. Gastropoda (HENNEGUY). Ova of Helix may be fixed 

 for from four to six hours in Mayer's picro-nitric solution 

 ( 58). The carbonate of lime that encrusts the external 

 membrane is thus dissolved, and the albuminous coat of the 

 gg is coagulated. The egg is opened with needles, the 

 albumen comes away in bits, and the embryo can be removed. 

 'Treat with successive alcohols, and imbed in paraffin. 



615. Limax (early stages) (MAEK, Bull. Mus. Comp. Zool., Harvard 

 'Coll., vi, 1881). The ova are treated with acetic acid of 1 to 2 per cent, for 

 four or more hours. The two external membranes are incised with fine 

 scissors, and the egg squeezed out in its albumen membrane. This is dis- 

 sected off on a slide, the egg is separated from the albumen, stained, and 

 mounted in glycerin. 



For later stages, or for making sections, osmic acid is used instead of 

 acetic acid, and the egg is hardened within its albuminous coats. 



HENCHMAN (Bull. Mus. Gomp. Zool., Harvard, xx, 1890, p. 

 171; Journ. Boy. Mic. Soc., 1891, p. 274; Zeit.f. wiss. Mik., 

 viii, 2, 1891, p. 216) finds that the best way of obtaining em- 

 bryos is to keep some twenty-five or thirty adults (of L. 

 maximus) in a large tin pail with a cover perforated with 

 small holes. They should be fed on cabbage, and the vessel 

 kept very clean. Eggs are generally found in the morning 

 in bunches of thirty to forty. As they are more abundant in 

 the early stages of confinement it is better to obtain a few 

 slugs often than many at once. In a moderately warm room 

 hatching occurs between the twenty-second and twenty- 

 seventh day. The eggs must be carefully protected from 

 desiccation. 



Kill with 0'33 per cent, chromic acid, or with liquid of 

 Perenyi. It is best to remove only the outer envelope before 

 putting into the chromic acid, the inner membrane being 

 removed after two or three minutes therein. Where Perenyi 

 is used the membranes must be removed first, as the albumen 

 will else coagulate in such a way as to prevent the removal of 

 the embryos. 



See also SCHMIDT, Studien zur Entwickelungsgesch. d. Pul- 

 monaten, Dorpat, 1891. 



