DEVELOPMENT OF THE OVUM. 



335 



Through a micropyle the spermatozoon finds entrance, 

 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, 

 surrounded 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 for a 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 



FIG. 165. 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 ; p. , posterior end of blastoderm ; 

 a.f., in the left-hand figure, the beginning of the amnion fold; 

 a.m., amnion ; a.c., amniotic cavity; s., serosa ; ec., ectoderm; 

 //., lower germinal layer; y., yolk. The amniotic cavity marks 

 the future ventral region of the embryo, so that the yolk mass 

 is dorsal. 



may be left behind in the central yolk to form the yolk nuclei, or, what 

 is probably the more primitive condition, these are formed by subse- 

 quent immigration from the blastoderm. 



The next process is the appearance of differentiation among the similar 

 cells of the blastoderm. Over a special area the ventral plate (cf. 

 Astactis] the cells increase in number and become cylindrical in shape ; 

 over the rest of the egg the cells flatten out and become much thinner. 

 In the middle of the ventral plate a slight groove is formed by rapid 

 multiplication of the cylindrical cells. This represents the disguised 

 gastrulation, the open roof of the groove being the much elongated 

 blastopore. The surrounding cylindrical cells unite over this open roof, 



