EMBRYONIC DEVELOPMENT 



819 



begins to pass down the oviduct. There it is met by 



ascending 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. 



The connection between embryo 

 and mother. ^«) The lowest Mam- 

 mals, the Duckmole {Ornithorhynchiis) 

 and the Porcupine Ant-Eater {Echidna), 

 resemble 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. exhibit m.eroblastic seg- 

 mentation like the eggs of Birds and 

 Reptiles. The tunic formed round 

 about them in the Graafian follicles of 

 the ovary consists, as in Birds and 

 Reptiles, of a single layer of cells. 

 Development begins in the 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. 



{b) In the Marsupials the embryo is 

 born prematurely after a short gesta- 

 tion. Itis very small and helpless. Till 

 recently it was believed that during its 

 intra-uterine life it was either not "^at- 

 tached to the wall of the uterus at all, 

 or only to a slight extent by a yolk-sac 

 placenta. It is now known, however, 

 that, in Perameles at least, there is not 

 only an efficient yolk-sac placenta, but 

 a distinct, though small, allantoic pla- 

 centa. The general absence of a placenta in Marsupials, 

 and the small size of the allantois, must therefore be 



Fig. 492. — Develop- 

 ment of hedgehog. 

 Three early stages. — 

 After Hubrecht. 



I. Shows iiiternal vesicle of 

 cndoderm ; the disc and 

 external sheath of ecto- 

 derm. II. Shows villi 

 arising from trophoblast ; 

 the disc of formative ecto- 

 derm (Ep.) ; the blasto- 

 dermic vesicle (B.t'.). III. 

 A more advanced stage : 

 Tr., trophoblast ; Ep., 

 disc of formative ecto- 

 derm ; Bv., blastodermic 

 vesicle ; H., cndoderm. 



