8o8 



DEVELOPMENT. 



[CH. LVIII. 



anterior end. It is due to a thickening of the epiblast, the cells 

 of which multiply so that they are several layers deep. This 

 appearance is called the primitive streak. 



This is soon marked by a narrow groove along its centre, which 

 is called the primitive groove (fig. 619.) 



If we cut a transverse section through the ovum at this period, 

 we have the appearance presented in the next figure (fig. 620). 



Fig. 623. Vertical section of area pellucida and area opaca (left extremity of figure) of 

 blastoderm of a fresh-laid egg. S, epiblast; D, hypoblast; 31, large "formative 

 cells," filled with yolk granules, derived from the hypoblast, ; A, the yolk 

 immediately underlying' the segmentation cavity. (Strieker.) 



The actual preparation is from a chick's egg ; the cicatricula 

 has divided into a number of cells, and these are arranged in two 

 layers, epiblast and hypoblast, but these, instead of surrounding 

 the whole ovum, lie spread out on the surface of the yolk. This 

 part of the embryo is subsequently pinched off from the yolk-sac. 

 The area which actually gives rise to the embryo is called the 



Fig. 621. Transverse section through embryo chick (26 hours), a, epiblast ; 6, mesoblast ; 

 e, hypoblast ; d, mass of cells at primitive streak ; , primitive groove. (Klein.) 



germinal disc, and the area opaca in the middle, seen at the left 

 extremity of the figure, is the opacity produced by the primitive 

 streak, and this is seen to be caused by the proliferation of 

 epiblastic cells, so that at this point they come into close contact 

 with the hypoblast. 



Fig. 621 shows a rather later stage ; this mass of cells, chiefly 

 epiblastic (d), gives origin to the intermediate layer or meso- 

 blast (b) which grows between the other two layers. 



