320 PERIPATUS, MYR10PODS, AND INSECTS. 



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 

 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 subsequent 

 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. 

 Astacus] 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 



FIG. 104.- 



-Diagram of Insect embryo, 

 and HEIDER.) 



(From KORSCHELT 



A transverse section before union of amnion folds, and a longitudinal 

 median section after union of amnion folds ; a, anterior pole of 

 ovum ; a', anterior end of blastoderm ; am, amnion ; s, serosa ; a.c, 

 amniotic cavity ; /, posterior pole of ovum ; ec, ectoderm ; //, lower 

 germinal layer ; y, yolk. 



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, 

 the groove usually flattens out, and thus we have formed a two layered 

 germinal streak which spreads forwards and backwards over the egg, 

 and early exhibits externally transverse division into segments. The 

 upper layer is the ectoderm, the lower includes the rudiments of both 

 mesoderm and endoderm. 



