726 



MAMMALIA. 



The spermatozoa are formed from germinal epithelium 

 in the testes. The primitive male cells or spermatogonia 

 give rise by division to daughter cells 

 or spermatocytes, which, with or 

 without further division, form sperma- 

 tozoa. 



The homologue of the ovum is the 

 spermatogonium or mother sperm cell, 

 but the physiological equivalent of 

 the ovum is the spermatozoon. 



The ovum, having burst from the 

 ovary, is immediately caught by the 

 fimbriated mouth of the Fallopian 

 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 sperma- 

 tozoa enters the ovum, and sperm 

 nucleus unites with ovum nucleus in 

 an intimate and orderly manner. 



The connection between embryo 

 and mother. (a) The lowest Mam- 

 mals, the Duckmole (Ornithorhyn- 

 chus) and the Porcupine Ant-Eater 

 (Echidna), resemble Birds and most 

 FIG. 396. Develop- Reptiles in bringing forth their young 

 ment of hedgehog. as e ggr Sj i.e. in being oviparous. The 

 are large with a considerable 

 quantity of yolk, and after fatuisa- 

 -3W tion divide partially, i.e. exhibit mero- 

 externai sheath of epibiast. blastic segmentation like the eggs of 



II. Shows villi arisine -r^. , 1-^ ., m, . r 



from trophobiast ; the disc Birds and Reptiles. The tunic formed 



epibiast rrmn ^ oKnnf tVi^m in thp> rirasfi'in 

 astodermic r 



in. A follicles of the ovary consists, as in 

 Birds and Reptiles, of a single layer 

 Development begins in the 



. , - r 



oviducts, but the eggs are in no 

 way attached to the wall. They are 

 laid in a nest by the Duckmole ; in the Echidna they are 

 hatched in a slight, periodically developed, external pouch. 



of formative 

 (); the blast 



vesicle (B.v.). 



. 

 disc of formative epibiast; o f cells. 



Bv., blastodermic vesicle; 



H. t hypobiast. 



