MAM M AL I A N E M BRYOLOGY 



In the mammal it is the amnion which is of secondary importance. 

 The yolk-sac has no yolk in it and in so far as we know has little func- 

 tional value. The allantois has but little respiratory and excretory sig- 

 nificance. Its work is practically to bring the embryo in relation to its 

 food supply. It is the chorion which becomes the principal organ by 

 which nutritive material and excreted substances between maternal and 

 embryonic circulations take place. 



It is this connection between mother and embryo which brings about 

 the formation of what is called the placenta. All mammals which de- 

 velop a placenta that is, all mammals except those which lay eggs 

 are known as placentals. 



THE PLACENTA 



The -detailed development of mammals must be left to much larger 

 volumes than this one, especially as there are so many variations occur- 

 ring even in quite similar groups of animals. 



D. 



Fig. 359. 



Diagrams illustrating the development of the blastocyst and formation of the 

 placenta in Mammalia. A, a blastocyst at the end of segmentation ; B, an older 

 blastocyst, in which a cavity has appeared to one side of the inner mass of cells ; 

 C, a. later stage, showing the formation of the trager and growth of the yolk 

 epithelium round the yolk cavity ; D, formation of lacunae in the trager and 

 commencement of the embryo ; E, further development of the trager, the mesoblast 

 has split and the amnion and extra-embryonic coelom are formed ; F, longitudinal 

 section of uterus, showing the position of the embryo in a pit in the uterine wall. 

 G. longitudinal section of a later stage, showing the obliteration of the old 

 lumen and formation of a new lumen in the uterus, all, allantois ; am, amniotic 

 cavity ; COB, embryonic ccelom ; ec, epiblast ; eec, extra embryonic coelom ; ek, 

 embryonic knob ; em, embryo ; gl, uterine glands ; hy, hypoblast ; im, inner mass of 

 cells ; lac, lacunae in trager ; lu 1 , original lumen of uterus ; ht 3 , secondary lumen 

 of uterus ; nch, notochord ; ng, neural groove ; sbm, thickened sub-mucous layer 

 of uterus ; tr, trager ; tro, trophoblast ; yk, yolk-sac : yk.e, yolk epithelium. In 

 all the figures the trophoblast is shaded with dots, and the embryonic mesoblast is 

 represented in black. (After Bourne.) 



