The Microscope. Ill 



uppermost, so that they may be readily seen without disturbing the 

 eggs. 



The first eggs should be removed after about eighteen hours of 

 incubation — when the first two layers of the blastoderm are well 

 defined. After thirty-six hours the three layers, epi, meso, and 

 hypoblast may be seen. 



The eggs of forty-eight hours, three, four and five days, should be 

 preserved. Older embryos, up to the seventh or eighth day, should be 

 obtained for the study of individual organs. As soon as one egg is 

 removed from the nest or incubator, another may be put in its place, 

 and thus in the course of a week or ten days a sufficient number of 

 embryos may be obtained to furnish work for the odd hours of many 

 months. During incubation the drawers should be frequently 

 opened and the covering cotton removed, in order that the eggs may 

 get plenty of air. 



§ 5. The embryos of reptiles, amphibians, mammals and fish 

 may be obtained as follows: 



Turtles deposit their eggs in holes which they dig in the sand 

 along the margin of streams and ponds. The time for laying 

 is about sun-down; the period of the year, from the second week 

 in June, varying with the locality. The eggs are hatched some time 

 in September or October. As soon as a turtle is seen to deposit its 

 eggs these should be carefully removed and planted in a box of 

 earth, covered by wire-gauze or mosquito-netting. They must be 

 kept moist. Incubation is very slow, so that an interval of several 

 days should elapse between the examinations of the embryos. 

 Turtles lay a large number of eggs, so that if a nest is found 

 material will be plenty. 



The eggs of frogs and toads may be found about the first of 

 June in streams and ponds. The former occur in glairy, gelatinous 

 masses; the latter in glairy, transparent strings. They should be 

 placed in shallow dishes of water, and portions removed and 

 hardened from day to day as the embryos develop. These latter 

 appear as black spots in the centre of the transparent globules. 

 The eggs hatch into tadpoles in about six or eight days. 



Fish eggs: These and the embryos can best be obtained at a 

 hatchery, but when this is inaccessible a spawning, adult fish may be 

 utilized. '* When a female fish is in fit spawning condition," says 

 Francis, "the vent becomes slightly enlarged and of a reddish 

 tinge. The ova, previously attached together by a membrane, 

 become disconnected. Take up the fish and hold it, first, head 



