544 EMBRYOLOGY OF THE LOWER VERTEBRATES ch. 



interpreting them by what has been made out from the whole 

 embryo and the series of transverse sections. The most instructive 

 sections are those in or close to the sagittal plane. Fig. 238 shows 

 diagrammatically a sagittal section through the whole length of the 

 embryo, but it will of course be understood that, owing to the head 

 of the embryo having come to lie over on its left side while the 

 trunk region retains its original position, a section which is sagittal 

 in the head region will, in actual fact, be practically horizontal in 

 the trunk. 



The feature that dominates the section is the cerebral flexure — 

 the strongly marked curvature of the head region towards the 

 ventral side. The brain is of relatively enormous size : a distinct 

 dip in its roof marks the boundary between the thin-roofed rhomb- 

 encephalon which lies behind it and the region in front of it — the 

 cerebrum — which will give rise to mesencephalon, thalamencephalon 

 and hemispheres. 



The next instructive feature brought out by such a section is the 

 general relation of gut to yolk-sac. The rounded head-fold of the 

 splanchnopleure has extended far back so as to floor in the foregut 

 (f.g). The velar membrane (v.m) has just ruptured so that the fore- 

 gut communicates in front with what will become the stornodaeum 

 into which also opens the pituitary involution of the ectoderm (jot). 

 The floor of the foregut dips downwards to form the rudiments of 

 the thyroid (th) and lung (I). In a slightly more advanced embryo 

 the two liver rudiments would be seen also as pocket-like outgrowths 

 of the enteric floor in the neighbourhood of the atrial end of the 

 cardiac tube. 



The posterior end of the definitive alimentary canal is also 

 becoming folded off from the yolk-sac though the cavity of the yolk- 

 stalk — the communication between the definitive alimentary canal 

 and the cavity of the yolk-sac — is still very wide. The position of 

 the future anal opening is indicated by a thick septum (a) composed 

 of fused ectoderm and endoderm. Dorsal and posterior to this the 

 enteron extends back as a blindly ending pocket — the remains of the 

 postanal gut (jpa.g), while anterior to the anus the enteric floor dips 

 downwards as the rudiment of the allantois (all). The latter is 

 covered with a thick layer of mesoderm and bulges into a dilated 

 portion of the splanchnoeoele. Towards the front end of the embryo 

 a still more widely dilated portion of the splanchnoeoele accommodates 

 the cardiac tube. At its anterior (v. A) and posterior ends (at) this 

 is ensheathed in the thick mesoderm on the ventral side of the fore- 

 gut, while its middle portion ( V) hangs free' in the cavity. 



Finally the amniotic fold of the somatopleure is seen to 

 extend almost completely over the body of the embryo, the 

 amniotic edge (a.e) bounding a comparatively small opening near 

 the tail end. 



Having studied in some detail the features characteristic of an 

 individual third-day embryo it will be convenient now to give a 



