THE INSECTS Iip 



fairly thick. In recent years, the importance of the cortex in other eggs, 

 such as those of the echinoderms, the spiral-cleaving forms, and the 

 Amphibia, has come to be recognised, and with this recognition, the 

 conditions in the insect egg begin to seem less pecuHar than they had 

 done previously. Instead of their organisation being fundamentally differ- 

 ent from that of other groups, wc see that they differ only quantitatively, 

 in that their cortical plasma is rather thicker than usual, and their yolk more 

 evenly distributed within the interior cytoplasm. 



Insect eggs often exhibit an obvious bilateral symmetry, the dorso- 

 ventral plane being built into them during their formation in the ovary. 

 They also tend to be provided with rather rigid external membranes, 

 through which a micropyle leads the sperm in to the egg. The vitelline 

 membrane, formed by the egg itself, is often partly chitinised and highly 

 impermeable to most solvents, a circumstance which makes fixation 

 difficult and renders the eggs troublesome objects for detailed study. 



Although as we have seen the structure of the insect egg is not entirely 

 dissimilar to that of other groups, it is nevertheless peculiar enough to 

 cause profound modifications in the cleavage. Insect eggs, in fact, do not 

 cleave at all in the normal sense of the word. After fertilisation the 

 gamete-nuclei conjugate somewhere in the neighbourhood of the micro- 

 pyle, and the zygote nucleus which is thus constituted divides a consider- 

 able number of times without any accompanying division of the main 

 mass of egg cytoplasm. Each nucleus, however, is surrounded by a small 

 region of relatively non-yolky cytoplasm and at each nuclear division, 

 this divides into two. The nuclei, accompanied by their patches of cyto- 

 plasm, move away from one another as though repelling each other, 

 and usually go through a stage in which they are arranged roughly as a 

 hollow sphere, or at least a hoUow shape more or less conformable to 

 the outline of the whole egg (Fig. 8.1). 



After some time, most of the nuclei reach the cortex and pass into it, 

 their cytoplasmic halos fusing with it. It is not until the cortex becomes 

 populated with nuclei that cell boundaries make their appearance, and 

 they form only in the cortex itself, which thus becomes converted into an 

 epithehum enclosing the main mass of yolk (which still contains a few 

 nuclei, whose later function is to take part in the digestion of the nutritive 

 materials). The enclosing epithehum is known as the blastoderm, and it is 

 from this that the embryo develops. 



The first sign of the embryo is a thickening of the blastoderm. This may 

 at first be double, so that there are two thicker strands, with many nuclei, 

 rather like the embryonic bands which form in an annehd which develops 

 without any larval stage, such as Tubifex (p. 96). Soon, however, the 



