Jl 



between tin- cnibrvo :uul aliantois (c, /'i, commensurate pressure is prcxluced by 

 iici-iiinuliitiiHi of fluid in these two jitiints or jioles of tin- circulation. This, to- 

 gether witli tin 1 action in tin- mtmihranes themselves, and the heart and vessels, 

 is siillicient forcaiTvine; on circulation in the iaitiul stages of embryonic evolu- 

 tiou ; lnit witli tin- incr. "\\tli comes increasing difficulty for effecting it; 



hence the pumping :ictii>n whicli is set up in the placenta and womb, together 

 with tin' accumulation of iimiiiotic lluid for transmitting these actions upon the 

 embryo, a.- described above. Tlnis, even tiling is in correspondence the 

 liquor iimuii, the inoreaoBg growth of tin- placenta .-mil the number of muscles 

 and nerves in the walls of the womb and so continues till the close of preg- 

 nancy. In other \vonls, it all forms a coniu-ctc d whole in the mechanics of the 

 i-mhryoiiie circulation. 



In tlic case of the bird, the jillantois answei-s to the placenta, since it pumps 

 in both o\\L,'en and nutriment ; only that the pumping action in the abdomen 

 (the soft hinder parts of the bird) for pumping air and blood through the 

 alveoli, is set up in the hitter days of incubation, when rapid atrophic changes 

 soon obliterate the umbilical vessels, and, breaking the now attenuated and 

 fragile sht 11-wall witli its bak, it finally makes its escape, leaving the allantois 

 and atrophied membranes behind. 



MODE OF GRAFUMi Till. o\l\l l\ THE TISSUES OF THE WOMB. 



In jiHiiiiiH'iliii tho ovum is not discharged from the maternal passages, but i 

 retained in the expanded oviducts at the point ol juncture, which answers to- 



FIG. 12. A VERTICU Sn Tins FROM THE FIG. 13. Same tubules, greatly magnified. 



UTERINE Mi'cors MEMBKAXK, showing the (After Dalton.) 



numbers and position of the tubules. 

 A, free surface ; 6, attached surface. 



(After Dalton.) 



the womb, where growth and elaboration are effected through the vascular con- 

 nections established between it and the maternal circulation. Briefly, the mode 

 of closing this is as follows : The uterine mucous membrane is virtually but a 

 dense mass of single, straight follicles, arranged perpendicularly to the free sur- 

 face (Figs. 12, 13), and with the closed end resting against the muscular 



