SEGMENTATION OF THE OVUM OF THE SHEEP. 251 



the greater portion of nourishment is obtained from the yolk 

 contained in the hypoblast cells. 



The diflFerence in size between the fertilised ovum of a reptile 

 or bird, and of a mammal is very great; but the difference in 

 size between the embryo of, say, a bird, with one pair of raeso- 

 blastic somites, and of a mammal of the same age, is compara- 

 tively small. This means that nearly the same space is 

 required for the production of the mammalian embryo as of 

 the Sauropsidan, and has to be provided. 



As the egg diminished in size, owing to the loss of yolk, a 

 corresponding increase in activity of the cells of the wall of the 

 vesicle must have taken place to keep up the size of the vesicle. 

 The smaller the egg became, the longer would be the time 

 occupied in the attainment by the vesicle of the requisite size. 

 Now, as the epiblast plays the more prominent part in the 

 formation of the bulk of the embryo during the earliest stages, 

 it clearly would be useless for the embryonic part to exhibit 

 much energy of growth until the old conditions were to a 

 certain extent regained ; hence the lethargy exhibited by the 

 embryonic epiblast in mammals during the first week of 

 development. No feature of the early stages of the mammalian 

 embryo is more striking than this inertness of the embryonic 

 epiblast — or, as I should now prefer to call it, simply epiblast 

 — during the first few days. 



A need for the extension of the edges of the epiblast as a 

 protective layer, as in the Sauropsida, has been obviated by the 

 development among mammals of the strong zona radiata; and 

 any tendency which existed for it to spread may very easily be 

 conceived to have been prevented by the increased activity of 

 the hypoblast cells, now no longer laden with yolk. This in 

 many cases resulted in a temporary overflow — as it were — 

 round the epiblast, aided perhaps originally, if not actually in 

 present embryos, by the restricted confines of the zona radiata. 

 Some such intermediate stage is shown in fig. D on PI. 18. 



It is probable that for the more primitive history of the 

 relation of trophoblast to the uterus and placenta we ought 

 to turn to the smallest mammals; and it is there that we see 



