. The development of the chick; an introduction to embryology . pr str. Fig. 63. —Einliryo of 12 s, from above, drawnas a transparent object with transmittedlight. X 30. Abbreviations as before. IV. The Mesoblast The changes in the mesobhist during this period are of greatimportance. At the time of appearance of the head-fold it con-sists of two great sheets of cells between ectoderm and entoderm no THE DEVELOPMENT OF THE CHICK beginning on each side of the heatl-jM-ocess and piimitive streak,and extending laterally and posteriorly to the margin of thevascular area. The lateral margins
. The development of the chick; an introduction to embryology . pr str. Fig. 63. —Einliryo of 12 s, from above, drawnas a transparent object with transmittedlight. X 30. Abbreviations as before. IV. The Mesoblast The changes in the mesobhist during this period are of greatimportance. At the time of appearance of the head-fold it con-sists of two great sheets of cells between ectoderm and entoderm no THE DEVELOPMENT OF THE CHICK beginning on each side of the heatl-jM-ocess and piimitive streak,and extending laterally and posteriorly to the margin of thevascular area. The lateral margins at this time extend anterior tothe embryonic axis, so that the anterior margin of the mesoblastforms a curve with {\w coiicavitv directed Fig. 64. — Head of the same embryo frombelow. X 30. .Abbreviations as before. The mesol)last in the region in front of the ])riniitive streakis known as (idstnd fucsoblast, and in the region of the primitivestreak as }>r()st(>niial mesoblast; the latter is fused with the primi-tive streak. However, the distinction between the gastral andprostomial mesoblast is not of permanent significance, becausethe latter is being continually converted into the former as theprimitive streak undergoes separation into ectodeim, notochord,and mesoderm. Confining our account now to the gastial mesoblast: a trans-verse section across an embiyo in which the head-fold is formingshows a sheet of cells lying on (>acli side of the notochord betweenthe ectoderm and entoderm. It is several cells deep near thenotochord, and thins giadually peripherally (cf. Fig. 56). Thethicker portion next the notoclun-d is distinguished as the paraxialmcsohlast (vertebral plate) fiom the more peripheral portion orl
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