202 EMBRYOLOGY OF INSECTS AND MYRIAPODS 



(average about 150) at a time. The eggs accumulate in a mass on the 

 abdomen and are later deposited in the water. 



The aquatic, hemispherical egg, which is 0.7 mm. in diameter, is 

 enclosed in a thick chorion and is held to the substratum by a gummy 

 "anchor base" attached to its flatter ventral surface. The micropyles 

 are distributed laterally around the entire circumference of the egg. 

 When the egg mass is placed in water, a temporary gelatinous envelope 

 around each egg expands and pushes the eggs apart. Formation of the 

 egg, including its gelatinous covering, is completed within the panoistic 

 ovariole. The egg develops with its ventral surface toward the base of 

 the ovariole and is still a primary oocyte when extruded by the 

 female, with the nucleus in the first meiotic metaphase. The egg is 

 entirely filled with yolk, cytoplasm being visible only around the ventral 

 nucleus. 



Development of the embryo is complete in about five and one-half 

 months, but under natural conditions the nymph remains dormant over 

 winter and hatches about ten months after oviposition, the egg period 

 being from June until the following April. 



Maturation of the egg nucleus occurs at the midventer of the egg; two 

 polar bodies are given off, undergo no further division, and soon degen- 

 erate. Cleavage is of the superficial type characteristic of most pterygote 

 insects. There is evidence that the first six divisions are essentially 

 synchronous. The cleavage cells reach the periphery of the yolk at 

 random, beginning at about the 128-cell stage, but the concentration 

 becomes greatest on the ventral surface. Probably all the cleavage cells 

 reach the surface. 



The peripheral cells of the primary epithelium (blastoderm) are 

 widely distributed and do not form a continuous layer. Compound 

 "primary nuclear aggregates," which seem to represent the prospective 

 embryonic cells, appear in the yolk during this stage; they presumably 

 arise by clumping of cells that migrate inward from the periphery. A 

 total of 600 to 700 nuclei are present in the egg when differentiation of the 

 embryo occurs. The embryonic rudiment is formed at the midventer of 

 the egg, evidently by direct streaming of cells (including the primary 

 aggregates) from the yolk to form a ventral mass which is then modified 

 into a unilayered ventral plate, or disk. Above the disk a number of 

 "secondary nuclear aggregates" are left which seem gradually to become 

 dispersed in the yolk, their components separating and forming vitello- 

 phags. Most of the vitellophags, however, are derived directly from the 

 primary epithelium as individual cells which enter the yolk at the time 

 of ventral streaming. "Yolk cleavage" occurs later in development. 

 Embryonic paracytes are formed during rudiment formation and early 

 growth of the embryo. 



