PLATE 5 



EXPLANATION OF FIGURES 



21 Section through two embryos of a capsule abundantly supplied with yolk. 

 Nuclei of the primary entoderm cells are undergoing degeneration while these 

 cells are being engulfed by those of the secondary entoderm. At ect. the mes- 

 ectodermic cells are becoming flattened to form the ectoderm. X 730. 



22 Thick section of capsule in which the upper embryo is in the process of 

 gastrulation. The ectoderm cells have nearly closed over the entoderm. The 

 primary entoderm cells have been grasped by those of the secondary entoderm. 

 At the right three ectoderm cells are absorbing free yolk left in the capsule after 

 the entoderm had become replete. Ectoderm at the left contains yolk which 

 has already been partially transformed. X 730. 



23 Oblique frontal section of an embryo just after gastrulation has been 

 completed. Same features as in preceding figure. The ectodermic yolk begin- 

 ning to be transformed (cf. the two cells at the top). The cells from which the 

 intestine is mainly to be derived are shown {ent. 2) lying between the yolk-laden 

 cells and the undifferentiated anterior cell mass. On the lower right several 

 large nuclei with their cytoplasm are to be added to the ectoderm. X 730. 



24 Oblique sagittal section of a slightly later stage in which the mesodermic 

 cells have pushed backward between the posterior body wall and entoderm. Eye 

 pigment cell just differentiated. X 730. 



cap., capsule 



ect., ectoderm 



ent.l, primary entoderm 



ent.2, secondary entoderm 



e.p.c, eye pigment cell 



mes., mesenchyme 



mes-ect., mes-ectoderm 

 ph.ep.int., internal pharyngeal epithe- 

 lium 

 t.y., transformed yolk 

 y., yolk 



548 



