528 EMBEYOLOGY OF THE LOWEK VEETEBEATES oh. 



the A posteriorly are the vitelline veins already alluded to (v.v), 

 while the median portion (JI) — a straight tube passing forwards 

 beneath the foregut — is the rudiment of the heart and ventral 

 aorta. It will be noticed that the two vitelline veins when traced 

 backwards from the heart are seen to fit round the tunnel-like 

 opening of the foregut. In the forebrain region is seen the 

 downwardly projecting pocket of its floor — the infundibulum (Fig. 

 232, inf) — and extending back from this in the middle line the 

 notochord (TV). On each side of this posteriorly are seen the meso- 

 derm segments (m.s). 



In a slightly more advanced embryo with about fifteen mesoderm 

 segments the tucking in of the blastoderm under the head has 

 proceeded considerably further. The neural tube has become closed 

 in entirely except for the slit-like remnant of the rhomboidal sinus 

 posteriorly. The optic rudiments projecting prominently from the 

 forebrain on each side and beginning to be narrowed slightly at 

 their base give the brain a conspicuous T-shape. The wall of the 

 brain in its posterior region shows a series of puckerings one behind 

 the other marking it off into a series of what used to be called brain 

 " vesicles." Of these the anterior one, the largest and most distinct, 

 is destined to become the mesencephalon while those behind it enter 

 into the formation of the rhombencephalon. The latter are often 

 interpreted as vestiges of a once present segmentation of the brain, 

 but are regarded by the author of this volume as being adequately 

 accounted for by the active growth of the brain within its confined 

 space, aided possibly by the varying consistency of the mesenchj-me 

 outside it (see p. 101). 



On each side of the head region posteriorly, just in front of the 

 first obvious mesoderm segment, the rudiment of the otocyst has 

 made its appearance as a cup-like depression of the ectoderm. 



The heart, growing in length more rapidly than the neighbouring 

 parts of the body, has been forced into its characteristic bulging 

 outward on the right side. The first traces of haemoglobin are 

 making their appearance in the posterior portion of the vitelline 

 network. 



An important new feature becomes visible about this stage in 

 the form of a whitish line on the bulging roof of the splanchnocoele 

 on each side. The lines in front curve in towards one another, 

 meeting in front of the proamnion and sweeping back in a wide 

 curve on each side. This line is the first rudiment of the amniotic 

 fold. As the fold becomes more and more prominent it bends 

 backwards and inwards, arching over the head region, and towards 

 the end of the second day (Fig. 233) forming the amniotic hood 

 which ensheaths the head portion of the embryo. 



Many of the important details in the structure of the second 

 day blastoderm can only be made out by the study of series of 

 transverse sections. . In studying the stage now under consideration 

 it is advisable to begin with a section taken from about the middle of 



