NEUROPTERA AND COLEOPTERA 



307 



length (Fig. 252). As the embryo shortens, the pit deepens, and a mass 

 of cytoplasm appearing in the yolk gathers on its tip (Fig. 253). This is 

 the first appearance of the developing mycetom. Soon nuclei appear to 

 migrate into this mass to make up the mycetocytes. As the end of the 

 stomodaeum grows thin, the mycetocytes gather in a mass, forming a cap 

 over the tip of the stomodaeum (Fig. 254). This mass of cells soon 

 moves from the end of the stomodaeum to form a ring around the stomo- 

 daeum close to the mid-gut walls. Before the embryo hatches, this ring 



Fig. 256. — Brachyrhinus. Cross section of abdomen of eight-day embryo. {cbl) 

 Cardioblasts. {ect) Ectoderm, {g) Gonad, {mge) Mid-gut epithelium, {nc) Nerve 

 cord, {somm) Somatic mesoderm, {splm.) Splanchnic mesoderm. 



breaks up into separate masses of cells which migrate through the walls 

 of the mid-gut and come to lie outside the mid-gut in close proximity to 

 the walls of the stomodaeum. This cell mass, which is described more 

 fully in Chap. XI, carries symbiotic organisms. 



The yolk, at first a homogeneous mass containing a great many nuclei 

 and structureless masses of cytoplasm, after blastokinesis becomes 

 divided into compartments by a reticulum. This reticulum seems to 

 form a provisional mid-gut wall which separates the yolk from^ the 

 epineural sinus on the ventral side. On the dorsal side the provisional 

 mid-gut wall is formed by a part of the amnion (Fig. 255). After the 

 mid-gut ribbons have begun their development, they form along the 

 surface of the membrane enclosing the yolk (Fig. 251). 



