DEVELOPMENT OF THE AMERICAN AEEICATOR — REESE 2J 



opens ventrally to the yolk sac. The other structures shown in the 

 figure are not markedly different from what was seen in figure lid. 



Figure nf is about one-tenth the length of the embryo posterior 

 to figure lie. The chief differences here noticed are in the enteric 

 and ccelomic cavities. The former is no longer enclosed, a dorsal 

 fold in the entoderm being all that remains of the cavity that was 

 seen in the more anterior figures, while the latter is here reduced to 

 a narrow cleft between the somatic and splanchnic mesoblast. A 

 thickening of the mesoblast on either side of the notochord, espe- 

 cially on the left, represents a mesoblastic somite. The medullary 

 canal (mc) is more open than in the more anterior sections. 



For about one-third of the length of the embryo posterior to figure 

 11/ there is a gradual flattening, in a dorso-ventral direction, with 

 loss of the amnion, until the condition represented in figure ng is 

 reached. The most striking feature of this region is the great 

 thickness of the ectoderm (ec), which is still made up of scattered, 

 irregular cells. In the middle line, directly over the medullary 

 canal (here a nearly cylindrical tube), is a sort of break in the ecto- 

 derm, as though there had not been a complete fusion of the epi- 

 dermal layer when the nervous layer came together on the closure 

 of the medullary groove. This break in the ectoderm may be fol- 

 lowed back to the region of the primitive streak, and will be men- 

 tioned again. As has been noted, the medullary canal (mc) is 

 nearly circular in cross-section, and is closely underlaid by the noto- 

 chord (fit), which is several times the diameter that it was in more 

 anterior sections. The mesoblast (mes) is a comparatively thin 

 layer, intermediate in thickness between the ectoderm and entoderm. 

 It shows laterally a slight separation to form the body cavity. 



Figure nh is about ten sections posterior to figure 11^, and dif- 

 fers from it chiefly in that the notochord (nt) is continuous with the 

 lower side of the medullary canal (mc) , though still distinct from 

 the underlying entoderm (en). 



Figure nt, four sections farther from the head, shows the same 

 greatly thickened ectoderm (ec) with the same break (ec') in the 

 middle line. The section is posterior to the notochord and passes 

 through the anterior edge of the blastopore or, as it may now per- 

 haps better be called, the neurenteric canal. The cells of the medul- 

 lary wall are continuous with those of the entoderm. The meso- 

 derm (mes) is still distinct from the other germ layers. 



Figure it/ is the next section posterior to the one just described 

 and differs from it only in showing the actual opening of the neuren- 

 teric canal (nc) into the medullary canal (mc). The medullary 

 canal extends, with gradually diminishing caliber, for about fifteen 



