290 CHARLES R. STOCKARD 



Lastly, such an experimental study bears closely upon the 

 general questions of relationship between different blood cell 

 types. The time and place of origin of the different cells, and 

 the developmental relationship and powers of transmutability 

 existing between various sorts of blood corpuscles as well as the 

 endothelial lining cells of the vessel walls are all problems upon 

 which the experimental results discussed above may throw light. 



Each of these three divisions of the problem embraces an 

 extensive and often cumbersome literature which it would be 

 quite out of place to consider in detail at the present time. We 

 shall, therefore, only consider the bearing of the facts recorded 

 in the previous pages upon the opinions and positions maintained 

 by the more recent investigators of the origin and development 

 of blood and endothelium. 



2. The specific problems of blood and vessel formation in the bony 



fish 



It may be well to review first the special problems and ques- 

 tions involved in the development of the blood in Teleosts as 

 a group. According to Swaen and Brachet ('01), the meso- 

 blast in the middle and posterior regions of the trout embryo is 

 arranged in two parts, a median primary somite portion and an 

 outer primary lateral plate part. The lateral plate in the mid- 

 body region then divides off a portion immediately adjacent to 

 the somites to constitute the intermediate cell mass. Lateral 

 to this a second part of the lateral plate is separated off to form 

 the primary nephric duct. In the mid-region of the body 

 the somites become separated from the primary lateral plate 

 and the lateral plate pushes or grows towards the median plane 

 and gives off a keel shaped mass between the somites and hypo- 

 blast. This mass unites in the median plane with a similar mass 

 from the other side and here forms a large cell group triangular 

 in cross-section, the intermediate cell mass. In the posterior 

 region of the embryo a similar mass pinches away from the 

 primary lateral plate and becomes the posterior continuation 

 of the intermediate cell mass. 



