162 CHARLES R. STOCKARD 



In the first place, the writer cannot resist the impulse to 

 highly recommend that all students of haematogenesis and vas- 

 cular origin spend some time at least in a study of living 

 mesenchymal cells and their cytomorphosis. Such a study will 

 soon convince one of the great disadvantages under which an 

 investigator labors in attempting to solve the origin of blood 

 from observations on dead material in serial sections. The 

 problem becomes so simphfied and devoid of laborious unin- 

 structive technique that it seems almost superficial. One may 

 learn as much from the living yolk-sac in an hour of careful 

 study as in almost a week's perusal of sections. Most important 

 is the fact that certain things may actually be seen to occur that 

 sections could scarcely stimulate the mind to imagine. The only 

 disadvantage is that the worker may be led to wonder whether 

 so apparently simple a problem is actually of scientific impor- 

 tance. Fortunately this mental state is soon passed over on 

 realizing the necessary care and precaution which must be taken 

 in following the movement and changes in the Uving cells. 



Each cell must be recognized as a living complex and the ob- 

 server will realize the importance as well as the difficulties of 

 thoroughly understanding and interpreting correctly its mani- 

 fold changes and behavior. Material which to some extent 

 allows such a study is often available. The Fundulus yolk-sac, 

 however, is exceptionally adapted to this study on account of the 

 beautiful simplicity of its structure, as well as the remarkable 

 clearness with which each cell may be observed. 



An investigation of the Fundulus yolk-sac readily supplies a 

 crucial answer to the old question regarding the relation of the 

 blood vessel lumen to other body cavities and spaces. Ryder 

 ('84) was right in describing the blastocoel of the bony-fish as 

 remaining an extensive cavity for some time. This is the space 

 between the ectoderm and yolk and is identical and continuous 

 with the cavity which arises very early beneath the blasto- 

 derm and above the yolk periblast. Agassiz and Whitman ('84), 

 as well as Ryder ('84), Wilson ('90), and others, have identified 

 this correctly as the cleavage cavity, the blastocoel. Later in 

 development, the blastocoel extends over the yolk, forming the 



