726 MAMMALIA. 
The spermatozoa are formed from germinal epithelizm 
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 Portupine Ant-Eater 
(Echidna), resemble Birds and most 
Fic. 396. — Develop- Reptiles in bringing forth their young 
ment of hedgehog. as eggs, ze. in being oviparous. The 
Three early stages.— : ; 
After Habrecht’ eggs are large, with a considerable 
creepiest oO of yolk, and after fertilisa- 
“hypoblast; the disc and tion divide partially, z.e. exhibit mero- 
fo ane ail ones blastic segmentation like the eggs of 
from trophoblast; the disc Birds and Reptiles. The tunic formed, 
Cr) tha Wastin Tound about them in the Graafian 
p blastodermic : A : 
tile Pe, Mie follicles of the ovary consists, as in 
re advances a . . - 
Tr, trophoblast; 29., Birds and Reptiles, of a single layer 
disc of formative epiblast; of cells. Development begins in the 
Bu., blastodermic vesicle 5 . : 
H., hypob'ast. 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. 
