THE EMBRYOLOGY OF A TERMITE. 565 



EXPLANATION OF PLATE XXXI. 



Fig. 24. Ventral surface of egg, just after closure of amniotic cavity: a.e., 

 anterior end of embryo; yc, yolk-cells intensely stained; sc, serosal cells, large 

 and of light color. 



Fig. 25. Side view (optical section) of slightly older embryo, slipped back out 

 of its usual position. The black ectoderm appears thicker in such an optical 

 section than it actually is : j., surface of yolk-mass ; a.e., anterior end of embryo ; 

 ch., chorion ; am., amnion ; id. (mes.) under-layer, or mesoderm, extending forward 

 beneath ectoderm. 



Fig. 26. Ventral view of germ-band two or three stages older than Fig. 24, 

 unsegmented and without cephalic lobes. The germ-band at this age is usually 

 placed as in the next fig-ure. 



Fig. 27. Embryo like that in the last figure, in its usual position. Seen from 

 side : a.e., anterior end ; c/i., chorion ; am., amnion ; p.e., posterior end. 



Fig. 28. Unsegmented germ-band with cephalic lobes, just before the appear- 

 ance of segments. Same stage as that of next figure, Fig. 29. The embryo is 

 dissected off from the yolk, and drawn with lower (yolk) side uppermost : am., 

 amnion cells, along edges of band ; a.L, anterior triangular area, between cephalic 

 lobes. The mesoderm cells are large black masses. 



Fig. 29. Unsegmented germ-band in same stage as last. Side view, to show 

 position on yolk. The amnion is faintly seen as a row of small dots beneath the 

 chorion. 



Fig. 30. Cross-section through middle of germ-disc, at about the age of Fig. 

 13, perhaps slightly older: ttLn. (mes.) under-layer or mesoderm nucleus crowded 

 below the surface ; d.n., nucleus dividing to separate a cell below ; yc, yolk cells ; 

 yds., yolk bodies of all sizes, and two perforated with holes left by solution of oil 

 drops. (0.003 mm. thick.) 



Fig. 31. Cross-section through region of under-layer plug, u/./>. {mes.), the 

 mesodermal rudiment, at the stage of the germ-disc shown in Fig. 18; d.tt., 

 nucleus dividing to separate cell below ; my.i., inner opening of micropyle ; my.c, 

 penetrating canal of micropyle through chorion ; my.o., outer opening of micro- 

 pylar funnel ; ch., chorion ; p.yb., perforated yolk bodies. A large yolk-cell lies 

 under the middle of section. Yolk bodies are large. (0.003 mm. thick.) 



The following six sections form a series, illustrating the growth of the germ- 

 disc and the mesoderm ; also the origin and growth of the amniotic fold. Nuclei 

 diagrammatic, except the dividing ones. 



Fig. 32. Median sagittal section of disc at stage in Fig. 18 : am.t., amniotic 

 thickening of ectoderm ; ul.p. [mes.), under-layer or mesodermal plug; a.ec, ante- 

 rior end of ectoderm of disc ; yc, yolk-cells ; yb., yolk body. Four dividing 

 nuclei. (0.004 mm. thick.) 



