90 



CYTOLOGY 



CHAP. 



Doncaster (1907) for the parthenogenetic eggs of the saw-fly, Nematus 

 ribesii, and later by Schleip (1910) for Rhodites rosae, a gall-fly. 



A very peculiar phenomenon observed by Brauer (1894) in Artemia 

 also falls into this category. As we have already seen (p. 89), the normal 

 procedure is for only one maturation division to take place, without 



Fig. 42. 



Meiotic prophase in the sexual egg oi Branchipus (A-G), and in the parthenogenetic egg of Artemia (H-J). 

 (After Fries, A.Z., 1910.) A, young oocyte I. ; B, synizesis and leptotene stage ; C, zygotene, D, pachytene 

 stages ; E, F, G, contraction of the chromosomes into the definitive bivalents ; H, young oocyte I. ; I, late 

 prophase, chromosomes longitudinally split ; J, definitive chromosomes of the maturation division. 



reduction of chromosome number. As a rare exception, however, Brauer 

 found that a second division took place, again without reduction. The 

 two nuclei resulting from this second mitosis must be regarded as equiva- 

 lent to the nuclei of the mature egg and second polar body. The latter, 

 however, is not extruded from the egg, but remains close to the egg 

 nucleus and moves with it to the centre of the egg. Thereafter these 

 two nuclei act like the male and female gamete nuclei in fertilization. 



