APP.] STUDY OF BLOOD-VESSELS. 450 



IX. Examination of tlie later changes of the Embryo. 



For the later stages, and especially for the deve- 

 lopment of the skull and the vascular system of the 

 body of the chick, it will be found necessary to dissect 

 the embryo. This can be done either with the fresh 

 embryo or more advantageously with embryos which 

 have been preserved in spirit. 



If the embryos are placed while still living into 

 spirit a natural injection may be obtained. And such 

 an injection is the best for following out the arrange- 

 ment of the blood-vessels. 



Sections of course will be available for study, 

 especially when combined with dissections. 



X. Study of the development of the Blood-vessels. 



Observations on this subject must be made with 

 blastoderms of between 3040 hours. These are to 

 be removed from the egg, in the usual way {vide II. 

 A. and C), spread out over a glass slip and examined 

 from below, vide II. E. 



The blastoderm when under examination must be 

 protected by a coverslip with the usual precautions 

 against pressure and evaporation, and a hot stage 

 must also be employed. 



Fresh objects so prepared require to be examined 

 with a considerable magnifying power (400 to 800 

 diameters). From a series of specimens between 30 

 and 40 hours old all the points we have mentioned 

 in Chapter iv. p. 92, can without much difficulty be 

 observed. 



Especial attention should be paid in the earlier 

 specimens to the masses of nuclei enveloped in pro- 

 toplasm and connected with each other by proto- 



