THE DEVELOPMENT OF VERTEBRATA. 303 



developed as larvae, they close up the aperture of the tube, and, 

 changing to a chrysalis, lie dormant until the time for their ap- 

 pearance in a perfect state, when the chrysalis-shell is burst open 

 and the perfect cadd ice-fly takes to its wings. 



VERTEBRATA. 



Next to those of the common fowl, none of the eggs of Ver- 

 tebrata are so accessible, and to be had in such large num- 

 bers, as those of Turtles and Tortoises ; and, insomuch as they 

 do not require, for hatching, more than the natural heat of the 

 ground in which they are buried when laid, they are, of all eggs, 

 by far the most easily preserved in a healthy state during the 

 time of incubation. All that is required to obtain them is to 

 collect a number of turtles in early spring, before May, and keep 

 them enclosed in some shady spot where they can have easy 

 access to water and soft earth, and feed them well with fresh 

 herbage, such as plantain-leaves, lettuce, beet-leaves, &c., &c., 

 and in the course of time, usually in May and June, they may 

 be caught, at early dawn, digging holes in the earth with their 

 hind legs, and depositing therein their brood of eggs, and then 

 covering them up. 



As the eggs are required for study they should be taken out one 

 at a time, carefully, so as not to disturb the others in the least, 

 and, whilst held in the same position as when in the nest, 

 the shell removed at the upper side so far down as to expose 

 the whole yolk. The eggs of turtles and tortoises have an en- 

 velope of albumen, the so-called ivhite, very much like that of 

 birds; and in order to keep it, as well as the yolk, in shape, and 

 moist, during prolonged observation, it is necessary to sink the 

 egg in some kind of fluid. For a short period water will suffice ; 

 but, as it eventually produces an injurious effect, some denser 

 material, such as thin syrup, which does not react so rapidly, had 

 better be used. The fluid which is left after beef -blood has 

 coagulated is perhaps as good as anything that can be had. 



The first trace of an organization presents itself in the form 

 of a round, light-colored disc which lies close to the surface of 







