GENERATION AND DEVELOPMENT 697 



as the membrana granulosa ; at one part these are heaped up, 

 known as the discus proligerus, and within this mass the ovum 

 lies buried. The Graafian follicle makes its way to the surface 

 of the ovary, being enveloped in a two-layered cover formed from 

 the stroma of the organ, known as the theca folliculi. The cells 

 of the inner layer develop the yellow pigment, lutein, which 

 subsequently, on the discharge of the ovum, secrete a yellowish 

 pigment, which stains the cells and fills up the gap left by the 

 discharged ovum. 



The mature ovum is very small ( T ^ to ^ of an inch) , yet large 

 enough to be seen by the naked eye. It is therefore one of the 

 largest cells in the mammalian body, though infinitely smaller 

 than the eggs of birds and reptiles. There is a mammal, the 

 'duck-billed platypus' (Ornithorhynchus) , which produces eggs 

 the size of a hazel-nut, and the ancestors of all mammals had eggs 

 probably as large. The greater size of the eggs of reptiles and 

 birds is due to the quantity of yolk, or deutoplasm, they contain, 

 which in mammals is vtry small in amount, as the embryo of 

 the latter is only dependent upon the yolk-sac for nourishment 

 for a very brief period. 



The ovum is a typical cell ; it is spherical, more or less trans- 

 lucent, and contained within a membrane, the zona pellucida. The 

 contents, or protoplasm, of the cell consist of fatty and albu- 

 minous granules, known as yolk spherules, and lying in the proto- 

 plasm is a nucleus, containing one or more nucleoli. The nucleus 

 is spoken of as the germinal vesicle, the nucleolus as the germinal 

 spot. One more body is found in the protoplasm, an attraction 

 sphere, or centrosome. This latter is extremely small, but its 

 functions are of the utmost importance. 



When the rupture of the Graafian follicle occurs, the ovum is 

 flushed out, and at the same moment, according to Henson, 

 the fimbriated extremity of the Fallopian tube becoming erect, 

 grasps the ovary, and thus the escaping ovum is received into 

 its ' duct.' Probably the converging furrows found on the 

 plicated extremity of the Fallopian tube may assist in directing 

 the ovum to the ostium abdominale. If by chance the ovum be 

 not caught and carried away to the uterus as described, it may 

 fall into the peritoneal cavity and perish ; or if it has been already 

 fertilised, abdominal foetation may occur, the peritoneum acting 

 as a matrix in which the embryo may develop. The method by 

 which the ovum gains the Fallopian tube is not, however, settled. 

 There is some evidence to show that it may be discharged into 

 the abdominal cavity, and make its way into the Fallopian tube. 

 The experimental introduction of small objects into the pelvic 

 cavity has resulted in these being taken up by the tubes. In 

 animals which have had one ovary removed the embryo has 



