474 HISTOLOGY 



With the complete formation of the second polar body the sperm 

 head enlarges and becomes reticular to form the male pronucleus (Fig. 

 446, m.p.). At the same time the chromatin, and the part of the second 

 maturation spindle which remains within the ovum, become organized 

 into the female pronucleus (Fig. 446, /./>.). The blending of these two 

 pronuclei results in the nucleus of the fertilized ovum. 



Among many animal forms, most of them highly specialized, the 

 female gives birth to some of her young or lays eggs that hatch and de- 

 velop without the presence or aid of any male. This condition is com- 

 mon among certain insects and crustaceans as well as some other ani- 

 mals, and is known as parthenogenesis. 



The young so produced are not the only offspring of their parent, for 

 at other times certain young are derived from eggs that have been fer- 

 tilized by the male cell or spermatozoon, and which develop as usual. 



Sometimes all the parthenogenetic young are females, a state which 

 is termed thelytoky. In other cases only males result from the partheno- 

 genetic process, and this condition is termed arrenotoky. When both 

 males and females are produced, the process is known as amphotoky. 



The cytological question which at once suggests itself is, do such 

 offspring come from eggs, and if so, do the eggs mature and develop with 

 half the number of chromosomes or do they secure a full complement of 

 chromosomes in some other way? 



Investigation shows that the eggs do mature ; that sometimes they 

 give off two polar bodies and in other cases only one. As both these 

 cases occur in Artemia, the brine shrimp, and as the parthenogenetic 

 process has been worked out and understood in this form by Brauer, 

 we shall use a description of this form as a concrete example. 



The normal number of chromosomes in the cells of this animal is 168, 

 and that number exists in the earlier stages of the reproductive cells of 

 parthenogenetic females. At the time of maturation, these chromo- 

 somes become arranged as 84 tetrads, and in the ensuing division these 

 latter are separated, 84 dyads going to the first polar body and the 

 other 84 staying in the oocyte. 



The subsequent development shows two forms. In the second type 

 of Brauer (which we describe first because it seems more like the usual 

 methods), the remaining 84 dyads in the egg divide, and one set of 84 

 of the resulting chromosomes remains in place to form the egg nucleus, 

 while the other 84 pass to the surface to help form the second polar body. 



If the egg nucleus with its half number of 84 chromatin units were to 

 proceed to develop by cleavage, there would result an embryo and adult, 

 all of whose nuclei had but one half of the proper number of chromo- 

 somes. Before development begins, however, the second polar body 

 returns and unites with the egg nucleus, and then development proceeds 



