566 GAMETOGENESIS CHAP. 



increase immensely, swelling out the microscopic ovum 

 until it becomes what we know as the " yolk " of the 

 egg : around this layers of albumen or " white " are 

 deposited by the glands of the oviduct, and finally the 

 double shell-membrane and the shell. Hence we have 

 to distinguish carefully in eggs of this character between 

 the entire " egg " in the ordinary acceptation of the 

 term, and the ovum or egg-cell. But complexities of 

 this sort do not alter the fundamental fact that all the 

 higher animals begin life as a single cell ; or in other 

 words, that multicellular animals, however large and 

 complex they may be in their adult condition, originate 

 as unicellular bodies of microscopic size ; and the same 

 is the case with plants. 



Gametogenesis (Spermatogenesis and Oogenesis). In 



the preceding chapters it has more than once been 

 stated that sperms arise from undifferentiated epithelial 

 cells in the spermary, and that ova are produced by the 

 enlargement of similar cells in the ovary. Fertilisation 

 has also been described (p. 197) as the conjugation of 

 two different kinds of gametes, the ovum and sperm 

 (compare p. 573). We have now to consider in greater 

 detail what is known as to the precise mode of develop- 

 ment of sperms (spermatogenesis) and of ova (oogenesis), 

 as well as the exact steps of the process by which an 

 oosperm or unicellular embryo is formed by the union 

 of the two sexual elements. 



Both ovary and spermary are at first composed of 

 cells of the ordinary kind, the primitive germ- or sex-cells ; 

 and it is only by the further development of these that 

 the sex is determined. 



In the spermary the sex-cells (Fig. 150, A) undergo 

 repeated fission, forming what may be called the sperm- 

 mother-cells (B) or male gametocytes (pp. 287 and 348). 



