36 ARTFIUU. DblNDY. 



which is really part of the somatopleure. Both these layers 

 are clearly shown in figs. 63 and 64, from which it will also be 

 seen that there is no recognisable mesoblast between the two. 

 Behind the vitelline veins, however, the true amnion envelops 

 the dorsal portion of the embryo, and is composed of somato- 

 pleure alone, with distinct epiblast and mesoblast, the latter 

 becoming continuous with the mesoblast of the serous envelope 

 in the mid-dorsal line (figs. 68 — 70). 



Although it was no longer conspicuous externally, the pos- 

 terior amniotic canal is still clearly recognisable in sections 

 behind the embryo. It appears as a very narrow canal (figs. 

 73, 73, P. A. C), embedded in the mesoblast of the serous 

 envelope, and extending obliquely backwards from the point 

 where the latter becomes disconnected from the true amnion 

 to a short distance beyond the sinus terminalis, where it still 

 opens to the exterior on the surface of the blastoderm. The 

 luiflen of the canal is distinct in the hinder part of its course 

 (figs. 72, 73), but in front it is closed, so that it is no longer 

 possible to trace it into connection with the amniotic cavity 

 above the embryo, as in earlier stages. The epiblast cells 

 lining the posterior amniotic canal seem to become gradually 

 assimilated to the surrounding mesoblast cells, so that it is 

 difficult, if not impossible, to recognise it at its extreme an- 

 terior end after its lumen has disappeared. 



External Characters of the Embryo. — In this par- 

 ticular specimen the head end of the embryo projects beyond 

 the anterior margin of the clear area of the overlying blasto- 

 derm, so that when viewed from above as an opaque object 

 part of the head is concealed from view by the yolk-laden 

 opaque area (fig. 56). When viewed from below as an opaque 

 object the embryo exhibits the appearance shown in fig. 57. 

 The fore-brain, with the optic vesicles, has been bent down- 

 wards at right angles to the long axis of the body, so that 

 what was formerly its anterior extren)ity now lies ventrally. 

 Immediately behind the fore-brain a dark-looking triangular 

 depression, with its apex pointing backwards, marks the 

 position of the stomodajum (fig. 57, Siotn.), where the ventral 



