616 PRACTICAL DIRECTIONS CHAP. 



stopped. Remove sufficient of the shell and shell-membrane 

 to expose the entire blastoderm, as well as the embryo 

 proper when developed. Examine with a hand-lens before 

 removing, and then, after carefully cutting through the 

 vitelline membrane with fine scissors around the margin of 

 the blastoderm, the latter should be floated off in a watch- 

 glass, and the covering vitelline membrane removed by the 

 aid of a needle an operation which requires considerable 

 care in the early stages. Fix with corrosive sublimate, and 

 after about half an hour wash in water and preserve in 

 increasing strengths of alcohol up to 90 per cent, (see p. 136). 



Two specimens of each stage should be preserved, one for 

 examining entire, and the other for sectioning l ; they should 

 subsequently be stained entire with borax-carmine or 

 haematoxylin, the one mounted in Canada balsam on a slide 

 after treating with absolute alcohol and xylol or oil of cloves, 

 and the other imbedded in paraffin (see p. 137). Serial 

 sections of each embryo should be mounted in order, after 

 smearing the slide with collodion and oil of cloves or gly- 

 cerine and albumen (p. 139). 



Sketch typical preparations of the various stages, both 

 entire embryos and sections. 



I. External characters. 



1. Unincubated egg. Segmented blastoderm (Figs. 149, 

 153, D). 



2. First day of incubation (18-20 hours). Note the area 

 pellucida, area opaca, primitive streak, and medullary plate 

 (Fig. 153, E). 



3. End of first day (about 24 hours}. Note the area 

 pellucida, area opaca, pro-amnion, head-process and medullary 

 groove, primitive streak and groove, and the small number of 

 mesodermal segments, which increase in number from before 

 backwards (compare Figs. 153, E, and 158, A). 



4. Second day. Note a, that the blastoderm has further 

 increased in size, that blood-vessels are apparent in the area 

 opaca, and that the head end of the embryo has become 

 raised above the yolk and is beginning to be covered by the 

 head-fold of the amnion ; b, the medullary folds, which meet 

 in the middle line except at the posterior end, in front of the 

 primitive streak, while in front the medullary cord thus 

 formed has become swollen to form the vesicles of the brain, 

 and the optic vesicles are seen standing out right and left 



1 The early stages are difficult to prepare, and the most im- 

 portant 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. 



