PRELIMINARY ACCOUNT. 343 



From the large size of the young rabbit at birth it might be 

 inferred that the rabbit's ova or eggs were large. This, however, 

 is not the case ; the egg of the rabbit is really very small indeed, 

 and, at the time it leaves the ovary, measures only 0*116 mm. in 

 diameter, i.e. is practically the same size as the egg of Amphi- 

 oxus, and only a fifteenth the diameter of the frog's egg. 



It is clear that this very small egg could not develop into an 

 animal the size of a rabbit at birth unless it received a very 

 plentiful supply of nutriment from without ; and inasmuch as 

 the whole increase in bulk takes place while the embryo is 

 within the uterus, it follows that there must be some arrange- 

 ment in the uterus for supplying it with food. 



This is effected by means of a special organ known as the 

 placenta, in which blood-vessels, derived from the embryo and 

 from the mother respectively, lie side by side in very close and 

 extensive contact with one another. The two sets of blood-vessels, 

 foetal and maternal, are distinct, but interchange of fluid and 

 gaseous contents takes place readily, by diffusion through their 

 thin walls, and in this way the blood of the embryo receives 

 nutrient matter from the blood of the mother during the whole 

 period of gestation. 



The placenta is formed practically by the allantois, which 

 develops as an outgrowth from the alimentary canal of the 

 embryo, and which at an early period becomes closely attached 

 to the wall of the uterus of the mother. The blood-vessels of 

 the allantois are, as in the chick, directly continuous with those 

 of the embryo ; and, by the outgrowth of vascular processes 

 of the allantois into the walls of the uterus, the vessels of the 

 embryo are brought into intimate relation with the large, 

 dilated, and very thin-walled blood-vessels of the uterus itself. 



Although the eggs of the rabbit, like those of nearly all 

 other Mammals, are very small indeed, yet in the mode of their 

 development they agree in several respects with the large eggs 

 of the bird, rather than with the smaller ones of the frog, or of 

 Amphioxus. A large yolk-sac, for instance, is formed from the 

 rabbit's ovum, although it contains no food material whatever ; 

 the embryo appears in the middle of the blastoderm, and there 

 is a well-marked primitive streak present ; while the formation 

 of the amnion and allantois are further points of resemblance 

 with the chick, and of difference from the frog or Amphioxus. 



