PLATE 4 



EXPLANATION OF FIGURES 



16 Section through an embryo to show clearly the three germinal regions, 

 mes-ectoderm, primary and secondary entoderm. X 730. 



17 Section through a capsule of which the two embryos have begun to ab- 

 sorb yolk. This process is carried on by the secondary entoderm cells; each 

 shows the mitochondrial mass and several yolk granules. X 730. 



18 Section through part of a capsule in which the secondary entoderm cells 

 have wandered to the posterior side of the primary entoderm. Most of the 

 yolk has been absorbed by the entoderm. X 730. 



19 Cell showing probable method of d'vision of the mitochondrial mass. 

 X 520. 



20. Mitochondrial mass after its first division. Centrosome inside at the 

 left, outside at the right. X 520. 



cent., centrosome ent.2, secondary entoderm 



d.y.n., degenerating yolk cell nucleus vies-eci., mes-ectoderm 

 ect., ectoderm mil., mitochondrial mass 



ent.l., primary entoderm y., yolk 



546 



