586 PRACTICAL DIRECTIONS chap. 



Vertebrate aiaimal, it is necessary to make a number of serial transverse 



sections. 



, For this purpose chick-embryos (see below) are, on the whole, the 



most convenient and satisfactory, but if you also wish to make sections 



of embryos of the frog, proceed as directed on p. 214. 



Serial sections of embryos in different stages should be mounted on 

 the same slide, after smearing it with collodion and oil of cloves (p. 139). 

 It is a matter of some difficulty to make satisfactory sections of the early 

 stages : the most important stages for the present purpose subsequently 

 to segmentation are from the time when the embryo begins to become 

 elongated up to hatching. 



A number of fresh, impregnated fowl's eggs should be obtained and 

 placed in an incubator at a temperature of from 37 to 40 C. or under a 

 ' ' broody " hen, first marking each with the date. One or two should 

 be examined each day or oftener for the first four or five days of incuba- 

 tion. To expose the embryo, place the egg in a dish of warm water 

 (temperature as above), in order that, after the first day, the beating 

 of the heart and the circulation of the blood may not be stopped. 

 With the forceps, tap the surface of the egg lying uppermost so as to 

 break the shell into small pieces, which can then be removed : cut 

 away sufficient of the shell-membrane with the scissors to expose the 

 entire embryo and blastoderm. The early stages are difficult to observe, 

 and the most important of those referred to below are from the end of 

 the first to the third day of incubation. As the medullary groove 

 only closes gradually from before backwards in the body-region, sections 

 showing different stages in the development of the central nervous system 

 may be obtained from the same embryo at these stages. Prepare as 

 above. 



1. Examine first an unincubated egg, as directed above, and make out 

 ts structure (compare Fig. 138). (The blastoderm undergoes seg- 

 mentation before the egg is laid. ) 



2. First day of incubation {\%-20 hours). Examine with a lens and 

 compare Fig 145 A. Then carefully cut round the blastoderm with fine 

 scissors, float it off in the water, preserve, stain, and mount entire in 

 Canada balsam. (The removal and preparation of the blastoderm at this 

 stage is rather difficult, and sections of the next stage will illustrate the 

 chief points equally well). 



