34 THE MAMMALIAN EGG 



nucleoli were reported to consist of a finely granular inner mass 

 surrounded by a thick zone of much denser material. The structure 

 was essentially the same as for oocyte nucleoli (p. 18) (Sotelo and 

 Porter, 1959). Hamster pronuclear nucleoli, on the other hand, did 

 not show the 'shell' when examined by electron microscopy (Fig. 

 27) although the method of fixation was similar. It may be that 

 nucleolar substance is prone to a physical change such as condensa- 

 tion under certain artificial conditions and in this state has a greater 

 affinity for osmium and some stains. 



Anomalies of Pronuclei 



Subnudei. In those eggs that are ovulated in the metaphase of the 

 second meiotic division, the chromosome group remains quiescent 

 until sperm penetration occurs or for 12 hr or more in the absence 

 of sperm penetration. In some unpenetrated eggs, the spindle 

 eventually regresses, however, and the chromosome group breaks 

 up or fragments, the chromosomes becoming scattered through the 

 egg cytoplasm (Fig. 28a). This course of events is well known in 

 the eggs of rats and mice and is commonly followed by the forma- 

 tion of a number, as many as twenty or thirty, of very small nuclei. 

 These are referred to as subnuclei; each is bounded by a nuclear 

 membrane and contains from one to several small nucleoli suspended 

 in a clear nucleoplasm (Fig. 28b, c). They can reasonably be re- 

 garded as being derived from isolated chromosomes, parts of 

 chromosomes or small groups of chromosomes. 



Clearly, however, the term subnucleus is arbitrary, for the nuclei 

 vary greatly in size and there is no doubt that there exists a more or 

 less continuous series of nuclei extending from simple, diminutive 

 forms to those resembling pronuclei of normal size and complexity. 

 As the size of the nuclei increases, the number that can be formed 

 decreases, so that at one end of the series the egg contains a pro- 

 nucleus-like near-diploid nucleus together with a small subnucleus — 

 a nuclear state not far removed from that seen in the initial phase of 

 one form of parthenogenesis when a single diploid nucleus may be 

 present. These facts suggest that eggs have an innate tendency 

 towards parthenogenetic development and such a view has often 

 been advanced. The nuclear state as thus described docs not, how- 

 ever, represent the whole situation. Eggs with fragmented nuclei, 

 especially those with numerous subnuclei, commonly show a 

 cytoplasmic state that is clearly abnormal and marks them as 



