456 THE PHYSIOLOGY OF REPRODUCTION 



While the foetal ectoderm advances along the vessels, it remains 

 stationary at the non-vascular parts. Hence there is an interlocking 

 of maternal and foetal tissues, and peninsulse of multinucleate cells 

 come to lie between the projections of the trophoblast. At the same 

 time the intermediary region decreases in thickness, and the ecto- 

 derm reaches the superficial sinuses of the deeper zone. Here the 

 uninucleate decidual cells again become multinucleate, apparently 

 at the expense of the blood symplasma, in the same manner as 

 formerly at the expense of the glandular symplasma. The sinuses 

 enlarge, and their walls proliferate into several layers of degenerated 

 cells, which after mid-term are gradually replaced by laminae of 

 fibrin. 



At a later period the intermediary zone still further decreases in 

 thickness, and the multinucleate cells gradually "melt to form a 

 granular detritus " (Duval). At the end of pregnancy the maternal 

 placenta consists almost entirely of blood and blood symplasma, 

 except for a thin rim of tissue containing blood-sinuses at the zone 

 of separation. The gestation period is thirty days. 



As compared with the placenta of Carnivora, it is obvious that 

 the dilatation of maternal vessels is much more marked in the 

 rabbit, and throughout the placenta the maternal blood is in direct 

 contact with the trophoblast, and not only at the border or round 

 a blood-pouch. The blood is not degenerated to a sufficient extent 

 to exhibit the varieties of pigmentation found in the zonary placenta. 

 Chipman does not state whether the maternal blood circulates in the 

 trophoblastic tubes, but Maximow l says that it does. Similarly 

 Duval says : " The maternal blood circulates from the foetal extremity 

 towards the maternal extremity of a lobule " (i.e. the series of tubules 

 derived from one tube). According to Masius, 2 " the maternal blood 

 circulates in an ectoderrnal mass of foetal origin." Herein lies a great 

 difference between the placenta of Rodents and Carnivores or Ungu- 

 lates. In the sheep the main nutriment is furnished by the glands ; 

 the maternal blood which is in contact with foetal ectoderm is 

 stationary and small in amount, and serves chiefly as a supply of 

 iron ; the exchange of gases takes place through foetal and maternal 

 tissues. In the dog the gland secretion is less important ; the blood 

 is again stationary and restricted to certain situations, and it shows 

 markedly degenerative appearances, but it is greater in amount, and 

 probably furnishes other substances besides iron . for the foetus ; in 

 the angioplasmode the maternal blood circulates and here the exchange 

 of gases is effected, but again both maternal and foetal tissues inter- 



1 Maximow, "Die ersten Entwicklungsstadien der Kaninchenplacenta," Arch, 

 f. mikr. Anat., vol. Ivi., 1900. 



2 Masius, "De la Genese du Placenta chez le Lapin," Arch, de BioL, 1889. 



