MAMMALIA. 



mother-sperm-cell, but the physiological equivalent of the 

 ovum is the spermatozoon. 



No one has succeeded in satisfactorily observing an 

 extrusion of polar bodies in the maturation of the mam- 

 malian ovum, but analog- 

 ous processes occur at an 

 early stage. 



The ovum having burst 

 from the ovary is imme- 

 diately caught by the fim- 

 briated mouth of the Fal- 

 lopian tube and begins to 

 pass down the oviduct. 

 There it is met by ascend- 

 ing spermatozoa, received 

 by the female as the result 

 of sexual union, and is 

 fertilised. One of the 

 spermatozoa enters the 

 ovum, and sperm nucleus 

 unites with ovum nucleus 

 in an intimate and orderly 

 manner. It is interesting 

 to remember that it was 

 only in 1843 that the union 

 of spermatozoon and ovum 

 was for the first time de- 

 tected by Martin Barry, 

 and in the case of the 

 rabbit. 



The Con?ieciio?i between 

 Embryo and Mother. — (a) 

 The lowest Mammals, the 

 Duckmole {Ornithorhy/i- 

 chus) and the Porcupine 

 Ant Eater {Echidna) re- 

 semble Birds and most 

 Reptiles in bringing forth 

 their young as eggs, i.e., in 

 being oviparous. The eggs are large, with a considerable 

 quantity of yolk, and after fertilisation divide partially, i.e., 



Fig. 224. — .Segmentation of Rab- 

 bit's ovum. (After V.^N Be.n'EDEN. ) 



€.c.. Extt^rn.-il cells (epiblast) ; i.e., in- 

 ti;rii.-il Culls (hypobLxst); Iw., W.istodermic 

 \ osiclc. 



