30 EMBRYOLOGY OF INSECTS AND MYRIAPODS 



sperm into a male pronucleus takes place at the same time as the forma- 

 tion of the female pronucleus. Although several sperms may have 

 entered the egg, only one male pronucleus unites with the female pro- 

 nucleus to form the fusion nucleus (synkaryon). 



The maturation spindles lack the asters, although centrosomes may 

 be either present as in Miastor (Kahle, 1908) or absent as in Apis 

 (Nachtsheim, 1913) and Lepidoptera (Seller, 1924). The first polar body 

 may again divide mitotically so that three polar bodies are formed, as 

 has already been described in Chap. I ; but in some cases it either does not 

 again divide, as in Pseudococcus citri (Schrader, 1923), or only partially 

 divides, as in Diacrisia virginica (Johannsen, 1929). With P. citri the 

 diploid chromosome group of the first and the haploid group of the 

 second polar body unite into a triploid group, which then divides mitoti- 

 cally, resulting in nuclei that become surrounded with cytoplasm and 

 thus give rise to giant cells which will later associate with the symbionts 

 of the egg. When the process has been completed, the giant cells are 

 known as "mycetocytes." 



During the first maturation division chromatin elimination may take 

 place in the eggs of a number of species of Lepidoptera (Seller, Johannsen, 

 1929), in Diptera {Miastor, Kahle, 1908; ^Scmra, DuBois, 1932), and other 

 forms. The elimination takes place in the form of an equatorial disk 

 (Fig. 295), the amount eliminated being variable, leaving a constant 

 amount in the daughter cells. 



Hegner (1917) in his account of the organization of the insect egg 

 concludes that the insect egg at the time of maturation is a mosaic of 

 differentiated cytoplasmic areas predetermined to develop into definite 

 parts of the embryo. This organization results from the interaction of 

 nucleus and cytoplasm during the germ-cell cycle. Such interaction is 

 taking place at all times but is visible only when such processes as the 

 protrusion of chromidia or chromatin diminution occur. 



The point in the egg where the maturation divisions take place is 

 not constant. In Simulium and some other Diptera and in Diacrisia it 

 is not far from the anterior pole; in Melophagus it is below the middle 

 on the dorsal side; in Pteronarcys it is mid ventral. It has been pointed 

 out that since the point of entrance and path of the sperm are probably 

 constant in any one species owing to the position of the micropyle, it is 

 uncertain whether or not they bear any relation to the origin of the 

 orientation of the embryo. The only cases in which any part of fertiliza- 

 tion is thought to have a bearing on development, other than initiating 

 completion of the maturation divisions, is in incompletely determinate 

 eggs. In this type of egg Reith (19316, 1935) reports that induction of 

 the visible zonation of the cortical layer of the ant egg and a similar but 

 not visible determination in the beetle egg start immediately after the 



