STUDIES IN MAMMALIAN EMBRYOLOGY. 505 



have only to count the number of cells that now compose the 

 blastocyst^ which we find at about fifty-six for the trophoblast, 

 twenty-six for the embryonic knob; whereas these numbers 

 were in the earliest phase thirty-six and nineteen for one and 

 thirty and seventeen for another embryo. 



In the sections of other embryos of about the same size the 

 embryonic knob has the appearance of being more bulky (figs. 

 10 and 11, PI. XXXVI). In the one case it, however, contains 

 twenty-two, in the other twenty-three cells ; the apparent dis- 

 crepancy is thus evidently occasioned by obliquity of the 

 section plane. Another somewhat older embryo, which 

 immediately precedes the stage in which the hypoblast is 

 going to separate from the inner cell-mass, shoAvs a further 

 increase in the trophoblast, with a stationary number of inner 

 embryonic cells. The stage alluded to (fig. 9) contains twenty- 

 two cells in its embryonic knob; the trophoblast is formed by 

 eighty cells, and as yet no coating of hypoblast-cells is detected 

 in any part of this blastocyst. 



In the following stage the hypoblast-cells are seen to spread 

 through the blastocyst, and at the same time the embryonic 

 knob is more flattened, and projects somewhat less into the 

 central cavity (figs. 22 — 24). There can be no doubt that 

 these large hypoblast - cells which gradually form a con- 

 tinuous layer clothing the trophoblast, and which then 

 constitute with it the didermic blastocyst, take their origin 

 from the embryonic knob. The cells of the latter are 

 less flattened and more bulky, the nuclei larger than those 

 of the trophoblast ; and by the latter peculiarity it is easy, 

 even before the hypoblast-cells form a continuous layer, to dis- 

 tinguish them from the trophoblast-cells against which they are 

 being applied (fig. 23). 



"With regard to the embryonic knob, two questions are not 

 without a certain importance : (1) Is there an indication that 

 the trophoblastic cell layer stretches above the embryonic 

 knob, or does it merge into this all along the border of the 

 latter ? 



The significance of this question will be understood when 



