158 



GENERAL EMBRYOLOGY 



equivalent to these processes in the Metazoa, but are merely divisions 

 by which a separation is effected between the reproductive and nutri- 

 tive chromatin, i.e., idiochromatin and trophochromatin; in nearly all 

 known forms only the former takes any active part in the subsequent 

 reproductive processes, while the trophochromatin usually dissolves and 

 disappears. 



Two very special modifications of the maturation process deserve 

 just a word. The first is in connection with those few eggs which 

 normally develop without fertilization (parthenogenesis), i.e., without 

 the union of equivalent egg and sperm nuclei. In such cases, which are 



FIG. 83. Maturation in the parthenogenetic egg of the brine-shrimp, Artemia. 

 After Brauer. A, X 795, others, X 368. A. Second polar body incompletely 

 cut off. B. Second polar nucleus reentering the egg and approaching the egg 

 pronucleus. C, D. Fusion of second polar body nucleus with egg pronucleus. 

 E. First cleavage spindle with two groups of chromosomes derived from the two 

 nuclei. II. Second polar body or nucleus; $, egg pronucleus. 



known in the Aphids, many Crustacea, and Rotifers, for example, the 

 normal course of maturation would lead to the formation of an organism 



with the haploid number of chromosomes ( |j in all of its cells. In 



most, if not in all, such cases which have been studied, it is now known 

 that as a matter of fact the egg is not left with the reduced number of 

 chromosomes. Thus in the brine-shrimp, Artemia (Brauer), which 

 illustrates the usual course of events in parthenogenesis, the first matu- 

 ration division proceeds as usual and is equational (reducing), leaving 



2 bivalent chromosomes in the secondary oocyte nucleus. Then one 

 of two courses may be followed (Fig. 83). In most normally partheno- 



