160 ETGHAED ASSHETON. 



Hypoblast. — (1) Hypoblast of embryonic area. This 

 seems to have become in very slight measure changed. It is 

 now undoubtedly a continuous membrane in the region of the 

 embryonic area. This condition seems to extend a distance 

 from the embryonic area equal to about tiie diameter of the 

 embryonic area^ beyond which it becomes a network and passes 

 insensibly into — 



(2) The straggling cell portion of hypoblast. This part of 

 the hypoblastic layer retains its irregularly scattered condition 

 of the sixth day, but certain features may be remarked upon 

 which were unnoticed before. 



The cells are more thickly scattered about; they are more 

 irregular, having entirely lost their rounded form, and are more 

 flattened. Many in all parts may be seen to be connected to- 

 gether by fine filamentous strands, not only in the close 

 proximity of the embryonic hypoblast, but also near its 

 periphery. 



Again, the outer limit of this zone is much more marked, and 

 is, in fact, now rendered very plain indeed. It forms a well- 

 marked edge, very irregular it is true, but an almost if not 

 quite continuous edge. Along this edge the cells are slightly 

 crowded, and rather elongated in the equatorial plane of the 

 vesicle. What I mean may be made out from fig. 40, and an 

 idea of the general history of events connected with the deve- 

 lopment of this part of the hypoblastic layer may be derived 

 from the four figures 37 — 40. 



The extent of area covered by the two parts of the hypoblast 

 is now rather more than half the whole area of the inside of 

 the wall of the blastodermic vesicle. 



Another point of interest may be noticed. There is a strong 

 tendency for this line of limit to be thrown into small folds or 

 waves, as shown in fig. 40, PI. 17. 



These two characters may be observed thoughout the area 

 under discussion. If an embryo of this age is cut in two, 

 and the cut edge examined under a high power, these characters 

 are seen very clearly. That is to say, the cells forming this 

 limiting line are themselves rather more rounded or *' hog- 



