CH.-VI] OOGENESIS 75 



Like the spermatozoa, the eggs are developed from small, 

 comparatively undifferentiated cells known as oogonia, but 

 not all the oogonia give rise to eggs, for many, in the 

 majority of animals at least, are used up in forming acces- 

 sory cells of various sorts. The process is especially well 

 seen in the ovaries of Insects, in which the ovary consists 

 of "tubes" or strands consisting of oogonia at the apex, 

 while lower down the tube all the stages of the growth of 

 the eggs and the differentiation of the accessory oogonia 

 into follicle or nutritive cells can be seen in regular order. 



In such an ovary the oogonia may be seen in division 

 near the apex; the mitosis is of the ordinary somatic type 

 and the chromosomes have the somatic number. After the 

 last oogonial division the cells begin to enlarge and become 

 oocytes, and the nucleus goes through precisely similar 

 stages to those seen in the spermatocytes of the same 

 species. The nucleus of the young oocyte, like that of the 

 spermatocyte, consists of a tangled network of threads ; from 

 this a spireme is formed in which the thread appears for a 

 time double, probably from the pairing of the reticular 

 threads side by side, but according to some by their thick- 

 ening and splitting. The spireme goes through the "bou- 

 quet stage," followed by synizesis, and from this emerge the 

 reduced number of strepsitene loops, just as in spermato- 

 genesis. The bivalent loops then contract to some extent, 

 giving rise to half the somatic number of fairly short, thick 

 threads, but do not, at this stage, proceed to form the 

 typical gemini of the heterotype division. In the insect 

 ovary (at least in many forms) an interesting differentiation 

 takes place at this stage. In the cells which will develop 

 into eggs the threads remain comparatively long, the 

 nucleus continues to enlarge, and the deposition of yolk in 

 the cytoplasm commences. In other cells, which up to this 



