SEGMENTATION OP THE OYUM. OP THE SHEEP. 249 



I have no doubt at all that the true epiblast takes some 

 part in the formation of the true amnion, but I am not at all 

 sure that it forms the whole ; in fact, I think it does not. 



Since I hold that the trophoblast is probably of hypoblastic 

 origin in all these forms, it follows that the true chorion in 

 mammals is formed by the fusion of the allantois with hypo- 

 blast (i. e. trophoblast), whereas the analogous membrane in 

 birds is formed by the fusion of allantois with epiblast. To 

 some this may appear to be a fatal objection, as it implies 

 that the false amnion of a bird is not strictly homologous 

 with the false amnion of the pig. 



To this I would reply, firstly, that the more primitive form of 

 amnion for mammals may be that which we find in the guinea- 

 pig or hedgehog, in which the parts which serve as false amnion 

 are developed quite apart from the production of the true 

 amnion. Prom these we get through stages such as are found 

 successively in the bat, pig, and rabbit, to a method of pro- 

 duction very similar to that of the bird, as the condition of 

 development became more and more alike. 



Secondly, the essential feature of the hypothesis is that the 

 epiblast never grows round the ovum in placental mammals, 

 and with the exception of the true amnion it takes no part at 

 all in the formation of any part of the foetal membranes. In 

 birds the epiblast grows round the ovum, ultimately forming 

 a firm wall to a sac which enables the all-important organ, 

 the allantois, to become rapidly and effectively developed and 

 extended. 



In mammals the sac is sufficiently well produced at an 

 earlier stage, and the conditions necessary for the growth of 

 the allantois are provided without need of the extension of the 

 epiblast. 



The epiblast cells which grow round the yolk in the bird do 

 not take any part in the formation of the actual embryo; their 

 function ceases at the time of hatching, and they are thrown 

 off or absorbed. 



The progress of the epiblast over the yolk in the bird is 

 described as being due to the multiplication of the epiblast 



