350 PHYLUM ARTHROPODA, 
few cases yolk is almost absent, as, for example, in the 
summer eggs of the Aphides, which are hatched within the 
body, and in some forms where the young are endoparasitic. 
The ovum is surrounded by a vitelline membrane, and also 
by a firm chitinous shell, secreted by the follicular cells, 
which is often sculptured in a characteristic manner. ‘This 
shell is pierced by one or more minute holes (micropyles). 
Through a micropyle the spermatozoon finds entrance, 
Fic. 184. Diagrams of Insect embryo.—After Korschelt and Heider. 
A transverse section before the union of the amnion folds, and a 
longitudinal median section after the union of the folds. a., 
Anterior end of blastoderm; Z., posterior end of blastoderm; 
af., in the left-hand figure, the beginning of the amnion fold ; 
am., amnion; @.c., amniotic cavity; s., serosa; ¢c., ectoderm; 
2., lower germinal layer; y., yolk. The amniotic cavity marks 
me ae ventral region of the embryo, so that the yolk mass 
lorsal, 
sometimes (as in the cockroach) after moving round and 
round the shell in varying orbits. 
The ripe egg usually consists of a central yolk-containing mass, sur- 
rounded by a thin sheath of protoplasm. As is usual in Arthropods, 
the segmentation is peripheral or centrolecithal. The central nucleus 
divides up into several nuclei, which, being united by protoplasmic 
cords, form fora time a central syncytium. Later, these nuclei emigrate 
into the peripheral protoplasm, which segments around them ; thus.a 
peripheral layer of similar epithelial cells is formed. Some of the nuclei 
