FORMATION OF THE BLASTODERM. 19 



streaJe (figs. 18, 1 9). Almost as soon as the primitive streak has become fully formed, it 

 may be observed to be scored along its length, except at the anterior end which is that 

 directed towards the centre of the embryonic area, by a narrow groove the 

 primitive groove. The proliferating primitive ectoderm comes into close relationship 

 below this groove with the primitive entoderm, and the two may be partly or 



^ 



*'\ m 



w 



Fig. 17. EMBRYONIC AREA OF MOLE IMMEDIATELY PRIOR TO APPEARANCE OP PRIMITIVE STREAK AND 



FORMED OF TWO LAYERS ONLY, 



Fig. 18. EMBRYONIC AREA OF THE MOLE SHOWING THE PRIMITIVE STREAK AND GROOVE ENDING 



POSTERIORLY IN A CRESCENTIC THICKENING. 



The area is bilaminar in front, trilaminar in the posterior half. 



Fig. 19. A SOMEWHAT LATER STAGE IN WHICH THE PRIMITIVE STREAK REACHES TWO-THIRDS OP THfl 

 LENGTH OF "THE EMBRYONIC AREA, AND ENDS BEHIND IN A KNOB OR THICKENING. 



Figs. 17, 18, and 19 are copied from Heaps. They are magnified 49 times. 



entirely blended, but the union is closest at the anterior end of the primitive streak 

 where a continuous column of cells unites the primitive ectoderm and entoderm, 

 so that the two layers are here indistinguishable. 



The proliferation of the cells of the primitive streak subsequently proceeds chiefly 

 at the sides of the primitive groove, and the cells which are produced by this proli- 

 feration extend themselves laterally between the ectoderm and entoderm to form a 



Fig. 20, A. and B. VIEWS OF THE EMBRYONIC AREA OF THE RABBIT SHOWING TWO STAGES IN THE 



EXTENSION OF THE MESO BLAST. (Kolliker.) 



In A. the mesoblast extends on either side of the primitive streak over the posterior part of the 

 embryonic area and also behind the primitive streak beyond the limits of that area. 



In B. the mesoblast extends over a circular area which surrounds the embryonic area. The em- 

 bryonic area is also trilaminar, except in the middle line in front of the primitive streak. 



C 2 



