48 DEVELOPMENT OF THE BLOOD-VESSELS OF THE CHICK. 



the blood-vessels by means of sections, though these come in as 

 a valuable confirmation of the other method. 



For the purpose of examination I have employed (i) fresh 

 specimens ; (2) specimens treated with spirit, and then mounted 

 in glycerine ; (3) specimens treated with chloride of gold for about 

 half a minute, and then mounted in glycerine ; and (4) specimens 

 treated with osmic acid. 



All these methods bring out the same appearances with 

 varying clearness ; but the successful preparations made by 

 means of the gold chloride are the best, and bring out the 

 appearances with the greatest distinctness. 



The first traces of the blood-vessels which I have been able 

 to distinguish in the pellucid area are to be seen at about the 

 thirtieth hour or slightly earlier, at about the time when there 

 are four to five proto-vertebrae on each side. 



Fig. i shews the appearance at this time. Immediately 

 above the hypoblast there are certain cells whose protoplasm 

 sends out numerous processes. These processes vary consider- 

 ably in thickness and size, and quickly come in contact with 

 similar processes from other cells, and unite with them. 



I have convinced myself, by the use of the hot stage, that 

 these processes continually undergo alteration, sometimes uniting 

 with other processes, sometimes becoming either more elongated 

 and narrower or broader and shorter. In this way a network of 

 somewhat granular protoplasm is formed with nuclei at the 

 points from which the processes start. 



From the first a difference may be observed in the character 

 of this network in different parts of the pellucid area. In the 

 anterior part the processes are less numerous and thicker, the 

 nuclei fewer, and the meshes larger ; while in the posterior part 

 the processes are generally very numerous, and at first thin, the 

 meshes small, and the nuclei more frequent. As soon as this 

 network commences to be formed the nuclei begin to divide. 

 I have watched this take place with the hot stage. It begins 

 by the elongation of the nucleus and division of the nucleolus, 

 the parts of which soon come to occupy the two ends of the 

 nucleus. The nucleus becomes still longer and then narrows 

 in the centre and divides. By this means the nuclei become 

 much more numerous, and are found in almost all the larger 



