534 EMBBYOLOGY OF THE LOWEE VEETEBEATES oh. 



On opening the egg it is at onee seen that the blastoderm has 

 increased considerably in size, the outer limit of the opaque area 

 having spread downwards as far as about the equator of the egg. 

 The vascular area has also increased considerably and is still 

 bounded by the conspicuous terminal sinus which anteriorly turns 

 inwards and passes back parallel to the corresponding part of the 

 sinus of the other side to open into the vitelline vein close to its 

 inner end. Of these two veins which run parallel to the long 

 axis of the embryo the right is reduced in size and eventually 

 disappears. 



The yolk has assumed a more fluid consistency ; the proportion 

 of white yolk has increased ; the albumen has shrunk considerably 

 in volume, and the air space has increased correspondingly. 



The free edge of the amniotic hood (Fig. 235, a.e) has grown 

 back so as to ensheath all the head and anterior trunk region of the 

 embryo. It follows that when examined in situ the front part of 

 the body is seen through two layers of somatopleure. Of these the 

 outer — the serous membrane — forms a kind of roof which passes 

 outwards all round into the general blastoderm. The inner — the 

 true amnion — closely invests the head end of the embryo and is 

 visible in profile as a sharp line immediately outside the outline of 

 the head itself. Anteriorly the amnion very often seems to be 

 prolonged into a sharp peak (Fig. 235, s.a) : this is the sero-amniotic 

 connexion. 



The free edge of the amniotic fold, somewhat arch-like in outline, 

 may die away posteriorly (Fig. 235) or it may be already continued 

 into the lateral and caudal parts of the fold (Fig. 236) — but even if 

 present these are still low and inconspicuous as compared with the 

 headward part of the fold. 



As regards the body of the embryo it is seen that the folding off 

 of this from the yolk is proceeding rapidly. The head and anterior 

 part of the trunk project freely and, correlated with this and with 

 the ventral flexure of the head region, the latter has come to lie 

 over on one side, usually the left, so that it is seen in profile when 

 the blastoderm is looked down upon from above. . At the extreme 

 hind end the tail region is also seen to be in process of becoming 

 marked off from the blastoderm by a tail fold (Fig. 235, t.f) of the 

 same nature as the head fold. Similarly the trunk region between 

 the regions of head and tail fold is becoming demarcated from the 

 blastoderm outside it by a lateral fold (Fig. 236). 



The body of the embryo has increased considerably in length and 

 this growth in length is particularly active towards the dorsal side 

 of the embryo where there is greater freedom from the clogging 

 effect of the yolk. The result of this difference in rate of growth 

 between dorsal and ventral sides is that those parts of the embryo 

 which are detached from the general blastoderm assume a strong 

 flexure towards the ventral side. This is particularly pronounced in 

 the head region, the head being completely bent upon itself so that 



