112 CHARLES R. STOCKARD 



irregular in shape and huge in size before their final degeneration. 

 After the embryo has hatched the remains of the yolk contain 

 a protoplasmic mass in which the periblast nuclei are packed 

 together and the whole is finally absorbed. 



The blastodisc is separated from the yolk by a space which 

 arises during the early hours of development. This space 

 between the ectoderm and periblast has been interpreted by 

 Agassiz and Whitman ('84), Ryder ('87), Wilson ('90) and others 

 to represent the blastocoel or segmentation cavity. It is actually 

 into this space that the wandering mesenchymal cells migrate 

 and we shall later recall this fact as of importance in interpreting 

 the nature of the vascular lumen in connection with other body 

 cavities or spaces. 



Twenty-four hours after fertilization the germinal disc is from 

 one-quarter to one-third way over the yolk-sphere. Its wall 

 has thinned out centrally and around the periphery is seen a 

 thickened border, the so-called germinal ring. After about 

 48 hours the germ-ring has traveled almost completely over the 

 yolk-sphere and now surrounds the small remaining uncovered 

 pole of the yolk which may be considered a yolk-plug. A shield- 

 shaped thickening, beginning about the twenty-fourth hour, 

 extends from one region of the germ-ring towards the animal 

 pole. This is the embryonic shield and along its median line a 

 second thickening begins to appear which is the first indication 

 of the embryonic body. 



From the edges of the embryonic shield and from the germ- 

 ring as it finally encloses the yolk, an early migration of meso- 

 dermal cells takes place. The cells apparently do not wander 

 far and some of them may again be included in the embryonic 

 body. After the germ-ring has enclosed the yolk, from 45 to 

 50 hours usually, a very active migration of mesenchymal cells 

 begins from the caudal and posterior lateral portions of the 

 embryo. Figure 1 shows outlines of a few such cells wandering 

 out from the side of an embryo of 45 hours. This figure is a 

 camera sketch from life and even at this early time there are 

 some cells inclined to be more or less spindle or stellate in shape 

 with long delicate filamentous projections, while other cells are 



