382 



ANNUAL REPORT SMITHSONIAN INSTITUTION, 1925 



produced are very small, while one maintains the normal size. This 

 large nucleus becomes the functional nucleus of the maturated egg 

 (Nu); the three small ones, known as the polat' ladies (/V>), are 

 rudimentary nuclei that move to the surface of the egg plasm 

 where they degenerate and are absorbed in the yolk. Only in ex- 

 ceptional cases do the polar bodies play a special part in 

 development. 



The egg is now ready for fertilization. The nucleus sinks back 

 again into the deeper part of the egg plasm and yolk, and the egg, 

 unless it is one of those capable of parthenogenetic development, 



has no further history until a sper- 

 matozoon unites with its nucleus. 

 In the course of normal events 

 with most insect eggs, however, a 

 spermatozoon has already entered 

 the egg plasm, or more probably 

 several have entered (fig. 8), so that 

 one is almost certain to accom- 

 plish its purpose. Before going on 

 with the subject of fertilization 

 we must, then, learn something of 

 what has happened to the sperm 

 cells during maturation, how they 

 have become transformed into sper- 

 matozoa, and how" the spermatozoa 

 gained an entrance into the egg. 



The original male germ cells 

 ordinarily contain the same num- 

 ber of chromosomes as the body 

 cells of the male insect that bears 

 them, but male cells in general, as 

 we have seen, differ from female 

 cells of the same species by having only one a?-chromosonie in the 

 nucleus, or in having a small y-chromosome as a mate for the 

 a?-chromosome. The male of a species in which the female germ cells 

 have six chromosomes would most likely have five chromosomes in 

 the nuclei of its germ cells (fig. 4, D), which would mean a chromo- 

 some formula of 4-f-la?. The single a?-chromosomo, having no mate, 

 remains single after synapsis (fig. 4 F), when the chromosome 

 formula of the male becomes 2+la?. At the first maturation divi- 

 sion (fig. 6, a), therefore, the a;-chromosome can not divide, and it 

 has to choose between going with one group or the other of the 

 orthochromosomes. As a consequence, one of the new nuclei will 

 contain an aj-chromosome, and the other will not. Two kinds of 



Fig. 6. — Diagram of chromosome divi- 

 sion during maturation of a sperm 

 nucleus (a) containing two double 

 orthochromosomes and one single x- 

 chromosome after synapsis (fig 4F). 

 The first division produces two nu- 

 clei (b), of which one contains two 

 single orthochromosomes and one x- 

 chromosome, and the other only two 

 orthochromosomes. The second divi- 

 sion produces four nuclei (c), two 

 with an x-chromosome, and two with- 

 out an x-chromosome. 



