112 EDWARD MCCRADY, JR. 



by coelom, but is continuous with the ventral body wall, which 

 is just in process of forming anterior to the cloacal membrane 

 (fig. 37). At this point of continuity with the body wall the 

 vessels of the allantois meet and empty into the umbilico- 

 postcardinal plexus already described in connection with the 

 posterior limb ridge. The developing allantois thus has a 

 venous connection with the umbilical veins at its very incep- 

 tion, but not with the vitelline or omphalomesenteric veins. 

 These latter vessels are at first almost exclusively vitelline, 

 and what few, small, mesenteric branches there are, are too 

 short to cross the wide mid-gut and so do not reach the al- 

 lantoic region. The subintestinal vein, which forms later and 

 acquires a connection with the allantois, will be described in 

 connection with stage 32. 



Miscellaneous details. The anterior neuropore completely 

 closes during stage 28. The reconstruction in figure 36 shows 

 the last vestige of it at the extreme anterior end of the 

 prosencephalon. The posterior neuropore, however, is still 

 in the form of a flat rhomboidal sinus (fig. 36, I). The first 

 traces of a marginal layer outside the mantle layer in the 

 central nervous system, indicate that axones and dendrites 

 are beginning to grow out from the nerve cells. However, 

 the cranial and spinal sensory ganglia are still unattached. 



The vascular system shows many new features. The capil- 

 laries which surrounded the otocyst in stage 27 have now 

 shifted the principal flow in the primary head vein to the 

 lateral side of the otocyst (fig. 36, A). Posterior to the oto- 

 cyst and anterior to the paracardinal plexus the head vein re- 

 ceives some rather large tributaries from the surface of the 

 central nervous system. These are the posterior cerebral 

 veins which play an important part in the formation of the 

 venous sinuses of the dura mater. A similar set just an- 

 terior to the otocyst constitutes the middle cerebral group, and 

 a third set from the mid-brain and forebrain is the anterior 

 cerebral group. These three groups become more sharply 

 demarcated from each other in later stages. The stub of a 



