220 PROCEEDINGS OF THE ACADEMY OF [Feb., 



Fig. 43 — Section of embryo which has sucked in a large amount of yolk (y^) 

 but is still surrounded by yolk, p^ = functional phar^mx closed. 

 & == a blastomere in embryonic layer, y^ = yolk-nucleus of embry- 

 onic layer. B. and L. 1-C 



Fig. 44. — Section of embryo from a capsule where all the yolk has been ab- 

 sorbed, embryo somewhat flattened. ]/ = yolk taken in by pharynx. 



Fig. 45.— Part of a section of an older embryo showing blastomeres filling 

 the embryonic layer, slightly thicker ventral side, and degenerating 

 embryonic pharynx (p'). 



Fig. 46. — Small part of section from same embryo showing dividing blasto- 

 meres or embryonic cells, very little yolk and few j^olk-nuclei (t/) 

 among the embryonic cells. B. and L. 3— C. 



Fig. 47. — Later embryo (4th day) showing a very early stage in the forma- 

 tion of the permanent pharynx (p^). e == ectoderm-cell containing 

 pigment and rhabdites. r = rliabdite-cell. B. and L. 1-C. 



Fig. 48. — ^Sections from an older embryo (6th day) showing later stage of the 

 pharynx (p-), well-developed ectoderm, and stands of embryonic 

 cells beginning to divide off the central yolk-region into axial gut 

 and its branches, a = endoderm-cells beginning to take in yollc- 

 cells. e = very young eye, optic cup of about 5 pigmented cells. 

 n = first appearance of nerve-cord. 



Plate XVI, Fig. 49.— The same as fig. 48. 



Fig. 50. — Another section of pharynx (p^) from same embrj'o. 



Fig. 51. — Section from same embryo, showing endoderm-cells (e) sending 

 out processes among the yolk-cells (//-). B. and L. i-C. 



Fig. 52. — Section from an older embryo (7-8 days) showing older eye (e), 

 larger nerve-cord (n) and formation of gut more advanced. 



Fig. 53. — Cross-section of 12th day embryo ready to hatch, showing well- 

 developed pharjTix (p-) and gut still filled with yolk-cells {y'^). B. 

 and L. 1-C. 



Fig. 54. — Another section from same embryo showing well-developed eyes 

 and brain (To) 



Figs. 55-56. — -Endoderm-cells containing large masses of yolk (y-), from 

 same embryo as Fig. 53. B. and L. x'— C. 



Fig. 57. — Similar endoderm-cell from a young planarian one day old, yolk 

 considerably disintegrated. 



Fig. 58. — Longitudinal section of young planarian, one day old, tail-region 

 shorter than in adult, (e = eye. h = brain, g = gut. p- = pha- 

 rynx B. and L. 1-a. 



